If you have many many many users that have filled out their birthday this step might be causing some problems. While I haven't run into it personally in all the imports I've done, you might just have a lot of birthdays and it's timing out on entering them all into the database. Come to think of it, this might be related to another bug report where birthdays still show up in the calendar when it's not supposed to show them. UBB.threads figures out birthdays by looking at the user profile table, they don't actually need to be inserted as an event.
What you should probably do is edit the classic_import.php file. Line 1866 of this file has:
$vars = array( "misc", "display", "registration", "calendar_events" );
You could change that to:
$vars = array( "misc", "display", "registration");
