diff --git a/pom.xml b/pom.xml
index 3166c6ec75..3ab3cb82a3 100644
--- a/pom.xml
+++ b/pom.xml
@@ -439,6 +439,13 @@
           <artifactId>smp-maven-plugin</artifactId>
           <version>1.0.0-alpha-6</version>
         </plugin>
+
+        <plugin>
+          <groupId>org.apache.maven.plugins</groupId>
+          <artifactId>maven-deploy-plugin</artifactId>
+          <version>2.8.2</version>
+        </plugin>
+
       </plugins>
     </pluginManagement>
 
@@ -633,7 +640,6 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-deploy-plugin</artifactId>
-        <version>2.7</version>
       </plugin>
 
       <plugin>
diff --git a/scm-core/src/main/java/sonia/scm/IllegalIdentifierChangeException.java b/scm-core/src/main/java/sonia/scm/IllegalIdentifierChangeException.java
new file mode 100644
index 0000000000..63b9d0d4fc
--- /dev/null
+++ b/scm-core/src/main/java/sonia/scm/IllegalIdentifierChangeException.java
@@ -0,0 +1,21 @@
+package sonia.scm;
+
+import java.util.Collections;
+
+public class IllegalIdentifierChangeException extends BadRequestException {
+
+  private static final String CODE = "thbsUFokjk";
+
+  public IllegalIdentifierChangeException(ContextEntry.ContextBuilder context, String message) {
+    super(context.build(), message);
+  }
+
+  public IllegalIdentifierChangeException(String message) {
+    super(Collections.emptyList(), message);
+  }
+
+  @Override
+  public String getCode() {
+    return CODE;
+  }
+}
diff --git a/scm-core/src/main/java/sonia/scm/repository/api/ModifyCommandBuilder.java b/scm-core/src/main/java/sonia/scm/repository/api/ModifyCommandBuilder.java
index 0dbd45d0a0..bddf9b378a 100644
--- a/scm-core/src/main/java/sonia/scm/repository/api/ModifyCommandBuilder.java
+++ b/scm-core/src/main/java/sonia/scm/repository/api/ModifyCommandBuilder.java
@@ -184,17 +184,19 @@ public class ModifyCommandBuilder {
     }
   }
 
+  @SuppressWarnings("UnstableApiUsage") // Files only used internal
   private File loadData(ByteSource data) throws IOException {
     File file = createTemporaryFile();
     data.copyTo(Files.asByteSink(file));
     return file;
   }
 
+  @SuppressWarnings("UnstableApiUsage") // Files and ByteStreams only used internal
   private File loadData(InputStream data) throws IOException {
     File file = createTemporaryFile();
-    OutputStream out = Files.asByteSink(file).openBufferedStream();
-    ByteStreams.copy(data, out);
-    out.close();
+    try (OutputStream out = Files.asByteSink(file).openBufferedStream()) {
+      ByteStreams.copy(data, out);
+    }
     return file;
   }
 
diff --git a/scm-ui/src/containers/Main.js b/scm-ui/src/containers/Main.js
index 8c05578b6b..5e982652e8 100644
--- a/scm-ui/src/containers/Main.js
+++ b/scm-ui/src/containers/Main.js
@@ -1,16 +1,16 @@
 //@flow
 import React from "react";
 
-import {Redirect, Route, Switch, withRouter} from "react-router-dom";
-import type {Links} from "@scm-manager/ui-types";
+import { Redirect, Route, Switch, withRouter } from "react-router-dom";
+import type { Links } from "@scm-manager/ui-types";
 
 import Overview from "../repos/containers/Overview";
 import Users from "../users/containers/Users";
 import Login from "../containers/Login";
 import Logout from "../containers/Logout";
 
-import {ProtectedRoute} from "@scm-manager/ui-components";
-import {binder, ExtensionPoint} from "@scm-manager/ui-extensions";
+import { ProtectedRoute } from "@scm-manager/ui-components";
+import { binder, ExtensionPoint } from "@scm-manager/ui-extensions";
 
 import CreateUser from "../users/containers/CreateUser";
 import SingleUser from "../users/containers/SingleUser";
@@ -34,7 +34,7 @@ class Main extends React.Component<Props> {
   render() {
     const { authenticated, links } = this.props;
     const redirectUrlFactory = binder.getExtension("main.redirect", this.props);
-    let url = "/repos";
+    let url = "/repos/";
     if (redirectUrlFactory) {
       url = redirectUrlFactory(this.props);
     }
@@ -44,9 +44,10 @@ class Main extends React.Component<Props> {
           <Redirect exact from="/" to={url} />
           <Route exact path="/login" component={Login} />
           <Route path="/logout" component={Logout} />
+          <Redirect exact strict from="/repos" to="/repos/" />
           <ProtectedRoute
             exact
-            path="/repos"
+            path="/repos/"
             component={Overview}
             authenticated={authenticated}
           />
@@ -67,9 +68,10 @@ class Main extends React.Component<Props> {
             component={RepositoryRoot}
             authenticated={authenticated}
           />
+          <Redirect exact strict from="/users" to="/users/" />
           <ProtectedRoute
             exact
-            path="/users"
+            path="/users/"
             component={Users}
             authenticated={authenticated}
           />
@@ -89,10 +91,10 @@ class Main extends React.Component<Props> {
             path="/user/:name"
             component={SingleUser}
           />
-
+          <Redirect exact strict from="/groups" to="/groups/" />
           <ProtectedRoute
             exact
-            path="/groups"
+            path="/groups/"
             component={Groups}
             authenticated={authenticated}
           />
diff --git a/scm-webapp/src/main/java/sonia/scm/api/v2/resources/SingleResourceManagerAdapter.java b/scm-webapp/src/main/java/sonia/scm/api/v2/resources/SingleResourceManagerAdapter.java
index a7b9146d00..76371a0b54 100644
--- a/scm-webapp/src/main/java/sonia/scm/api/v2/resources/SingleResourceManagerAdapter.java
+++ b/scm-webapp/src/main/java/sonia/scm/api/v2/resources/SingleResourceManagerAdapter.java
@@ -2,6 +2,7 @@ package sonia.scm.api.v2.resources;
 
 import de.otto.edison.hal.HalRepresentation;
 import sonia.scm.ConcurrentModificationException;
+import sonia.scm.IllegalIdentifierChangeException;
 import sonia.scm.Manager;
 import sonia.scm.ModelObject;
 import sonia.scm.NotFoundException;
@@ -11,8 +12,6 @@ import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.function.Supplier;
 
-import static javax.ws.rs.core.Response.Status.BAD_REQUEST;
-
 /**
  * Adapter from resource http endpoints to managers, for Single resources (e.g. {@code /user/name}).
  *
@@ -55,7 +54,7 @@ class SingleResourceManagerAdapter<MODEL_OBJECT extends ModelObject,
     MODEL_OBJECT existingModelObject = reader.get();
     MODEL_OBJECT changedModelObject = applyChanges.apply(existingModelObject);
     if (!hasSameKey.test(changedModelObject)) {
-      return Response.status(BAD_REQUEST).entity("illegal change of id").build();
+      throw new IllegalIdentifierChangeException("illegal change of id");
     }
     else if (modelObjectWasModifiedConcurrently(existingModelObject, changedModelObject)) {
       throw new ConcurrentModificationException(type, keyExtractor.apply(existingModelObject));
diff --git a/scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java b/scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java
index 5ae4a32897..fced7a01ed 100644
--- a/scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java
+++ b/scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java
@@ -59,6 +59,9 @@ import java.util.ArrayList;
 import java.util.Date;
 import java.util.List;
 import java.util.Set;
+import java.util.function.Predicate;
+
+import static java.util.stream.Collectors.toList;
 
 //~--- JDK imports ------------------------------------------------------------
 
@@ -68,6 +71,7 @@ import java.util.Set;
  *
  * TODO don't mix nio and io
  */
+@SuppressWarnings("squid:S3725") // performance is not critical, for this type of checks
 public final class PluginProcessor
 {
 
@@ -171,7 +175,11 @@ public final class PluginProcessor
 
     extract(archives);
 
-    List<Path> dirs = collectPluginDirectories(pluginDirectory);
+    List<Path> dirs =
+      collectPluginDirectories(pluginDirectory)
+      .stream()
+      .filter(isPluginDirectory())
+      .collect(toList());
 
     logger.debug("process {} directories: {}", dirs.size(), dirs);
 
@@ -194,6 +202,10 @@ public final class PluginProcessor
     return ImmutableSet.copyOf(wrappers);
   }
 
+  private Predicate<Path> isPluginDirectory() {
+    return dir -> Files.exists(dir.resolve(DIRECTORY_METAINF).resolve("scm").resolve("plugin.xml"));
+  }
+
   /**
    * Method description
    *
diff --git a/scm-webapp/src/main/resources/locales/de/plugins.json b/scm-webapp/src/main/resources/locales/de/plugins.json
index 92b81c5772..bf534dc138 100644
--- a/scm-webapp/src/main/resources/locales/de/plugins.json
+++ b/scm-webapp/src/main/resources/locales/de/plugins.json
@@ -167,6 +167,10 @@
     "CHRM7IQzo1": {
       "displayName": "Änderung fehlgeschlagen",
       "description": "Die Änderung ist fehlgeschlagen. Bitte wenden Sie sich an ihren Administrator für weitere Hinweise."
+    },
+    "thbsUFokjk": {
+      "displayName": "Unerlaubte Änderung eines Schlüsselwerts",
+      "description": "Ein Schlüsselwert wurde unerlaubterweise geändert."
     }
   },
   "namespaceStrategies": {
diff --git a/scm-webapp/src/main/resources/locales/en/plugins.json b/scm-webapp/src/main/resources/locales/en/plugins.json
index 3f70000624..471da10159 100644
--- a/scm-webapp/src/main/resources/locales/en/plugins.json
+++ b/scm-webapp/src/main/resources/locales/en/plugins.json
@@ -167,6 +167,10 @@
     "CHRM7IQzo1": {
       "displayName": "Change failed",
       "description": "The change failed. Please contact your administrator for further assistance."
+    },
+    "thbsUFokjk": {
+      "displayName": "Illegal change of an identifier",
+      "description": "A identifier value has been changed in the entity. This is not allowed."
     }
   },
   "namespaceStrategies": {
diff --git a/scm-webapp/src/test/java/sonia/scm/plugin/PluginProcessorTest.java b/scm-webapp/src/test/java/sonia/scm/plugin/PluginProcessorTest.java
index 0fbf7eabbc..bb518ec731 100644
--- a/scm-webapp/src/test/java/sonia/scm/plugin/PluginProcessorTest.java
+++ b/scm-webapp/src/test/java/sonia/scm/plugin/PluginProcessorTest.java
@@ -134,6 +134,16 @@ public class PluginProcessorTest
     assertThat(plugin.getId(), is(PLUGIN_A.id));
   }
 
+  @Test
+  public void shouldCollectPluginsAndDoNotFailOnNonPluginDirectories() throws IOException {
+    new File(pluginDirectory, "some-directory").mkdirs();
+
+    copySmp(PLUGIN_A);
+    InstalledPlugin plugin = collectAndGetFirst();
+
+    assertThat(plugin.getId(), is(PLUGIN_A.id));
+  }
+
   /**
    * Method description
    *