I have Forums running happily under 2.3.2. I loaded 'cms', 'jsparty' and 'sapphire' from 2.3.3 and then ran dev/build/?flush=1
All I got was a PHP error:
PHP Fatal error: Using $this when not in object context in . . . /forum/code/ForumRole.php on line 85
Reverting back to 2.3.2 fixed the problem.
Can someone confirm that the Forum module is OK in their set up under 2.3.3?