Well I PM you this
Reupload the default files for photopost do not make this edit I suggested as it was a test.
I install a default phpbb and things are great. Cookie expires a year from now. I would suggest in phpbb admin these values
Allow automatic logins
Determines whether users are allowed to select to be automatically logged in when visiting the forum
SET TO YES
Automatic login key expiry
How long a autologin key is valid for in days if the user does not visit the board. Set to zero to disable expiry.
I SUGGEST SOMETHING LIKE 7
These are new settings in phpbb and maybe this is your issue. Possibly you have autologin to off?