Module Load Failure with GWT 2.4.0 (but not with 2.3.0)

I use GWT with Maven and with GWT 2.3.0, "mvn clean gwt:run" works without problems.
When I upgrade to GWT 2.4.0 however, I get an "Module Load Error" (see linked image).

The GWT Version is controled by the following excerpt from my pom.xml:

    <dependency>
            <groupId>com.google.gwt</groupId>
            <artifactId>gwt-servlet</artifactId>
            <version>${gwtVersion}</version>
        </dependency>
        <dependency>
            <groupId>com.google.gwt</groupId>
            <artifactId>gwt-user</artifactId>
            <version>${gwtVersion}</version>
            <scope>compile</scope>
        </dependency>

gwtVersion is defined as either "2.3.0" or "2.4.0".
How can I get rid of the error while still using an up-to-date GWT?

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