The version of GWT in Errai 2.3 was updated to 2.5.1. The exception is thrown when using a lower version of gwt-maven-plugin for compilation (in this case 2.4.0). The versions should match.

The related property in the POM needs updating from

<version.org.codehaus.mojo.gwt.maven.plugin>2.4.0</version.org.codehaus.mojo.gwt.maven.plugin>

<version.org.codehaus.mojo.gwt.maven.plugin>2.5.1</version.org.codehaus.mojo.gwt.maven.plugin>

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