How to reproduce:
1. login to the tasklist as jonny1
2. create a filter with criterion candidateGroup and a value, which is not an expression ("foobar" will do)
3. edit the new filter
4. The criteria tab shows two criteria: candidateGroup and candidateGroups
5. Trying to save the filter (without changing anything) results in server error 500 (Invalid query usage: cannot set both candidateGroupIn and candidateGroup)
The filter contains the candidateGroup criterion only.