This is using vB
3.5.4 and Photopost 5.3.
I had this problem, and found that THIS fixed the problem:
update pp_settings set setting='bb' where id='172';
Execute this from a shell account, from within mysql. It's what I had to do- maybe I missed something somewhere, but this fixed it.
For the newbies, log in to your shell account via SSH (putty.exe or whatever), then log into mysql (mysql -uYOURUSERNAMEFORDB -pDBPASSWORD nameofyourdatabase), then type as above and press enter (assuming you used pp_ as your table prefix for Photopost), you should see "Query OK, 1 row affected". Be aware, typo's can blow up lots of things at this point.