Omegatron - I'm not sure, but this might be the problem. A user installed 4.8.6 and PHPBB and could upload etc. However, when we created usergroups for her new board and then went into Photopost admin under edit categories and clicked 'refresh usergroups from your forum software" that is when we started to have these problems.
The admin disappeared completely. I followed your other thread and made the changes to the showphoto and phpbb2 files. Now this is a new install of 4.8.6 so shouldn't they already have been fixed?
I hope this helps. I'm going into the table via PHPMyAdmin to fix that.