We have to admit, this is a pretty terrible error message. Sorry about that. We're working on a fix that will be much more helpful. It should say that we can't understand your query and tell you what part we found confusing.
In the meantime, here is a tip to solve it yourself:
The most common problem is that the Boolean query doesn't have a matched number of parentheses. Double-check that for every open parentheses "(" you also have a closed one ")" and see if that fixes the problem.
If you don't see anything wrong with your query, please copy the query and send it to us at the feedback link or at firstname.lastname@example.org. We'll check it out and reply to you with a version that works.