Error during compilation of the wigdetset: ... cannot be abstract

From time to time I get an error during compilation of the widgetset. I use for example the add-on
. Most of the time the compilation of the widgetset works without any problems. But sometimes I get the error:

[ERROR] Errors in 'com/allen_sauer/gwt/dnd/client/util/' [ERROR] Line 39: Rebind result 'com.allen_sauer.gwt.dnd.client.util.impl.DOMUtilImpl' cannot be abstrac Can anyone tell me, what is the problem? Why does it occurs “randomly”? Has anyone a solution for the problem?
I found a solution to resolve this problem, but it is very nasty. I have to clean all ivy caches. After a resolve the compilation works again.

An old thread but in case someone else stumbles upon this: I suspect there were two different versions of some GWT classes on the classpath. Perhaps one came as a transitive dependency of the add-on whereas the other is the version bundled in Vaadin.