Upgrading to the latest m2e-wtp definitely was the solution, also for this thread:
* I had a project without any project facets (due to m2e-wtp which didn't have the latest version)
* Then I tried to activate several project facets (Java, Web, JAX-RS, JPA, JSF, ...) manually -- this was obviously not a good idea...
* Afterwards the Eclipse workspace was unusable and caused a crash when I wanted to restart Eclipse.
Looks like m2e-wtp (or m2e ?) is a pretty sensitive plugin.