After hours of search and even exchanging eclipselink with hibernate, I was able to find out the reason. I wasnt smart enough to check the orm file(s).
The wiki actually states the example link for querying as:
...org.eclipse.emf.texo.web.example/jsonws?query=select e from Writer e
but actually it should be
...org.eclipse.emf.texo.web.example/jsonws?query=select e from library_Writer e
Thought you might be interested in this, since the mentioned thread above has no answer to the query problem.