When changing the user group for a user on a default install of 1.2.14 I immediately get an Internal Server Error. Unfortunately, I can't provide the relevant error_log, as the host I'm using does not provide it. With a little bit of testing, though, I narrowed down the problem to profile.php, line 1496:
<form id="profile7" method="post" action="profile.php?section=admin&id=<?php echo $id ?>&action=foo">For some reason, the "action=foo" is causing the ISE. When I change it to anything else (I used 'noaction'), it functions properly. This doesn't seem to occur on any other page that uses "action=foo", and I can't even reproduce it on my local machine.
It's entirely possible this is just a peculiarity of the configuration of the host I'm using, and not an actual bug/problem with punBB, but I figured I should post this, regardless.