When the import creates users, the new accounts don't have the private-messaging permission turned on.

This blocks import of private messages for all users except admin(s).

Instead, the code should take care to enable private-messaging either simply for all users imported from phpBB, or for all of those, who sent (write) or received (read) private messages.

The work-around is for the admin to manually add the private-message permission to all authenticated users after the import of users, but before the import of private messages.