Dear Ruben, the error occurs among other things when you try to release a calculated attribute in the OData service.
Such attributes do not exist in the database but are only calculated in runtime.
You should remove such attributes from your OData service.
Hope this helps.
The scheme value of category is always the same, as specified in the Atom Format for OData: http://www.odata.org/documentation/odata-version-3-0/atom-format/ (See section 6.1.8).
Do you see any errors in the mendix logfile that may point to the cause of your 500 error?