It's currently not possible to login with a user's password after direct conversion of phpBB 2.0.x to 3.3.x. The issues is caused by two parts:

• Updating hashes incorrectly uses the $CP$ as hash prefix which results in invalid combined hashes
• Combined hashes of phpBB2 passwords do not expect the conversion step of applying phpass to the phpBB2 md5 hashes and therefore do not check this during login