I have the problem, that the categories in my menue would lead to a non existing site and cause a 404 error when clicked. I have no other plugins installed besides GP premium and the latest WP version. What could cause this?
Try to delete the contents of your .htaccess file and save it.
After go to your Control Panel > Options > Permalinks
Update the permalink structure, this will generate a new rewrite rule for your permalinks.