[calorie]

The "you have been banned for the following reason" message is from the vB nopermission_banned phrase. The nopermission_banned phrase appears in the print_no_permission function found in the vB functions.php file:

	if (!($vbulletin->usergroupcache["$usergroupid"]['genericoptions'] & $vbulletin->bf_ugp_genericoptions['isnotbannedgroup']))
	{
		// snipped stuff here generates ban reason and lift date

		eval(standard_error(fetch_error('nopermission_banned', $reason['reason'], $date)));
	}

So for some reason your users are getting inside that if statement.

Maybe your usergroupcache needs to be rebuild. Try calling the following from your browser:

http://www.your-domain.com/your-forum-dir/your-admincp-dir/index.php?do=buildbitfields