It has been many years since we officially supported the original Mental Case app on iOS, but if you are still using it, you should install the latest Mental Case from the App Store now. The reason is that the original app will no longer run on iOS 11, due to restrictions placed by Apple.
To make sure you keep all of your data, download the new Mental Case app from the App Store. It should ask you to migrate your data when you first launch it. You should do this before you install iOS 11, because the original app cannot be installed or launched on iOS 11.
Finally, it is worth noting that we now have the Studies app, which replaces Mental Case. Once you have the latest Mental Case installed, and have migrated your data from the original app, you can install Studies (iOS) for free, and migrate to that, to be completely up-to-date.