That hack, of course, fixes the problem. However, Doctrine is failing to escape entities with reserved keywords in various different situations and this should be a major problem, specially since there are many keywords that are common table names. Having to change the table name or escape the table name manually is not the best solution.
I have look through the code but could not find out why getQuotedTableName() is failing to quote the table name "Group". I fixed the other problem, involving schema creation, but this one i couldn't fix. That's why i'm opening the issue, hoping someone with more experience in the ORM codebase manages to fix it.