This error comes up any time I try to group query results.
Here is my setup.
The error code (67 for HSQL v1.8) is labelled NOT_IN_AGGREGATE_OR_GROUP_BY so presumably there is something wrong with the manner in which you are trying to group. I will leave it to a Base expert to provide an answer.
Try adding aggregate functions to all of the columns. The way query is written now, if there is one value in eventTitle, and multiple in any of the other fields, query does not provide instruction of what value you want returned for other fields. MySQL would return only the first value in those columns, not display other values, which is not always obvious. Possibly HSQL returns an error instead?