A quick search here will pull up hundreds of threads on this subject.
It's always one of the following:
cookie_prefix
cookie_domain
cookie_path
vb3 license
Make sure you've entered your license number, not your customer number.
Make sure your prefix is correct,
VB by default uses "bb" for a cookie prefix.
You can view your current cookie settings in the config-inc.php file in your PhotoPost directory.