Could you please post the latest session row (from sessions table) of that user?
Error seems to be resolved after the user was manually activated by an admin.
d2a3a3f5714fff5cd79b2c6738837xxx 87 xx.xx.xx.xxx 1292703296 /online.php? Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) Ap... 0 0 0 0
Is the session row.
It seems $mybb->usergroup was not an array?
$mybb->usergroup = array_merge($mybb->usergroup, $mydisplaygroup);
First argument is $mybb->usergroup
I'd a contact a developer if I were you and point him/her to this thread.
Browse the usergroups table and printscreen what you see.