This is a symptom of CONF-5254, when duplicate labels exist on the system. An error like this may appear:

HibernateSystemException: query did not return a unique result: 2

The following query can be used to identify the offending labels:

select name, labelid from label group by name, labelid having count(name) > 1

and any duplicate labels can then be removed from the LABEL table. Relationships in the CONTENT_LABEL table will have to be mapped to the remaining label ID.