what gallery is this? Photopost is not affected by upgrading vbulletin from 3.5 to 3.6 series only 3.0 to 3.6 since only Photopost versions 5.21 and higher support vbulletin 3.5 and up
What version Photopost where you running?
I would suggest you check your config files to ensure all the proper information is in them. You can also set PHP display errors to ON so you can see the exact error or look at your servers error logs.