Should it be creating an integer timestamp to go in a date/time field? Dunno, it just doesn't seem to like that bit.
I've made a couple of changes so that it just copies the registration date from the phpbb user ID.
Changed line 373 to:
$regdate = date(' Y-m-d H:i:s', $row->regdate );
and added to line 486:
p.user_regdate as regdate
Seems to be working ok.