In a recent update, (5.6) WordPress changed some code which can affect your OptimizePress pages
We released a fix for our SmartTheme which should resolve this issue
The issue issue can be resolved by updating your SmartTheme version to the latest version. Go to Appearance > Themes on your site and you should see an update available for the SmartTheme
Once you update your site, this issue should be resolved. The issue was related to a recent change WordPress made in their framework which affected some code inside OptimizePress
You will need an active license in order to access the latest versions of OptimizePress plugins and themes.