Facing the same problem. I tried the nightly build, but it completely crashed my eclipse. Then i restored my backup and everything was ok during some time, and happened again. The only message at error log was a warning: "NLS missing message: J2EEModuleExportOperation_ErrorExportingArchive in: org.jboss.ide.eclipse.as.ui.Messages", but seems not related.
The "correct answer" above simply does not work for me. There are no process running, only eclipse.
Is there any kind of quick fix i can do? Now i'm running JBoss "from outside" eclipse.