Hostgator recently migrated all its customers to php5. So far I've found very few problems with this but I had an old dotproject version on one site, and began getting the error:

Fatal error: Call to a member function Execute() on a non-object in [...path to]query.class.php on line --"

Thanks to this post by Zanahade on the dotproject.net forums,the problem was easily resolved.

Click on System Admin>System Configuration, then look for the setting for "Session Handler". Change it to PHP Default.