Reference topic: https://www.phpbb.com/community/posting.php?mode=reply&f=466&t=2275221

If you grant a user the "Can lock own topics" permission, they can see the option in viewtopic. Clicking on the lock topic option brings up the confirmation window. After selecting "Yes", you receive an error message "You are not authorised to access this area". Users with moderator permissions are not affected.