I've got a message in my out-box that's been there since March. I can't delete it as it comes up with this message, similar to your's,
Could not update user pm counters
SQL Error : 1690 BIGINT UNSIGNED value is out of range in '(`retrobike_phpbb2`.`phpbb_users`.`user_unread_privmsg` - 1)'
UPDATE phpbb_users SET user_unread_privmsg = user_unread_privmsg - 1 WHERE user_id IN (16127)
Line : 853
File : privmsg.php
I had assumed the person I sent it to hadn't read it but maybe it's because he couldn't read it. Had something similar before, PM'd John, who sorted it for me. Didn't want to bother him over one PM but if you're having trouble too maybe we should let him know.