I would have to see your setup. You can email a url and admin login through the contact us link in our header if you want me to give an accurate account of what I can find.
Otherwise in the Photopost FAQ forum here there is a cookie tutorial. Your PhotoPost and UBB thread cookies should be set to the same thing meaning what your telling me that your ubbthreads needs to be the same. If your logging in no cookie is created that is being read sounds like your cookie settings are wrong.
These are the cookies we look for in Photopost right from the threads7 integration file.
Content visible to verified customers only.
Therefore what you need to do is login through the forum see what cookies are created and then clear cookies for your site and then login through the gallery and see the cookies created and compare.
A good browser for this is Mozilla. I can look at it if you send details.