See PCP-753 for a report of this problem by a customer.
The user reported that downgrading PC to 2.1.0-J7 solved the problem. This points clearly to the change in the queries that was released with version 2.2.0.
The problem is caused by the subquery optimization bug described here. There is also an excellent explanation in this page, in the section called "Badly optimized subqueries". It seems this bug was fixed with MySQL 5.6 (not verified).