Maven error after migrating from 7 to 8.0.0

Vaadin 6 / 7 / 8
1

Hi, I followed the instructions to migrate my project from vaadin 7.7.3 to 8.0.0 but I get an error trying to build the project:
I would be glad if someone could give me a hint or some help resolving the problem…

pom.xml → dependencies states "gwt-user 2.4.0 (managed from 2.8.0) [compile]
" in eclipse.

Stacktrace from windows-terminal: (mvn -pl testproject-vaadin clean install)

[INFO]
 --- vaadin-maven-plugin:8.0.0:compile (default) @ ps-test-disl-client-vaadin ---
[INFO]
 auto discovered modules [war.MyAppWidgetset]

[INFO]
 Using com.vaadin:vaadin-client-compiler version 8.0.0
[ERROR]
 Feb 27, 2017 12:01:47 PM java.util.prefs.WindowsPreferences <init>
[ERROR]
 WARNUNG: Could not open/create prefs root node Software\JavaSoft\Prefs at root 0x80000002. Windows RegCreateKeyEx(...) returned error code 5.
[INFO]
 Loading inherited module 'war.MyAppWidgetset'
[INFO]
    Loading inherited module 'com.vaadin.DefaultWidgetSet'
[INFO]
       Loading inherited module 'com.vaadin.Vaadin'
[INFO]
          Loading inherited module 'com.google.gwt.user.User'
[INFO]
             Loading inherited module 'com.google.gwt.resources.Resources'
[INFO]
                [ERROR]
 Unexpected error while processing XML
[INFO]
 java.lang.NoClassDefFoundError: com/google/gwt/core/ext/GeneratorExt

the pom.xml looks like this:

<dependency>
            <groupId>com.vaadin</groupId>
            <artifactId>vaadin-compatibility-server</artifactId>
            <version>8.0.0</version>
        </dependency>
        <dependency>
            <groupId>com.vaadin</groupId>
            <artifactId>vaadin-push</artifactId>
        </dependency>
        <dependency>
            <groupId>com.vaadin</groupId>
            <artifactId>vaadin-compatibility-client</artifactId>
                        <version>8.0.0</version>
<!--             <scope>provided</scope> -->
        </dependency>
        <dependency>
            <groupId>com.vaadin</groupId>
            <artifactId>vaadin-compatibility-themes</artifactId>
            <version>8.0.0</version>
        </dependency>
        <dependency>
            <groupId>com.vaadin</groupId>
            <artifactId>vaadin-themes</artifactId>
            <version>8.0.0</version>
        </dependency>

<build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-war-plugin</artifactId>
                <version>2.6</version>
                <configuration>
                    <failOnMissingWebXml>false</failOnMissingWebXml>
                    <!-- Exclude an unnecessary file generated by the GWT compiler. -->
                    <packagingExcludes>WEB-INF/classes/VAADIN/widgetsets/WEB-INF/**</packagingExcludes>
                </configuration>
            </plugin>
            <plugin>
                <groupId>com.vaadin</groupId>
                <artifactId>vaadin-maven-plugin</artifactId>
                <version>${vaadin.plugin.version}</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>update-theme</goal>
                            <goal>update-widgetset</goal>
                            <goal>compile</goal>
                            <!-- Comment out compile-theme goal to use on-the-fly theme compilation -->
                            <goal>compile-theme</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-clean-plugin</artifactId>
                <version>3.0.0</version>
                <!-- Clean up also any pre-compiled themes -->
                <configuration>
                    <filesets>
                        <fileset>
                            <directory>src/main/webapp/VAADIN/themes</directory>
                            <includes>
                                <include>**/styles.css</include>
                                <include>**/styles.scss.cache</include>
                            </includes>
                        </fileset>
                    </filesets>
                </configuration>
            </plugin>

            <!-- The Jetty plugin allows us to easily test the development build by 
                running jetty:run on the command line. -->
            <plugin>
                <groupId>org.eclipse.jetty</groupId>
                <artifactId>jetty-maven-plugin</artifactId>
                <version>${jetty.plugin.version}</version>
                <configuration>
                    <scanIntervalSeconds>2</scanIntervalSeconds>
                </configuration>
            </plugin>
        </plugins>
    </build>