M&M website works fine.
Lufthansa website works fine.
Lufthansa iOS App - cannot login
M&M iOS app - cannot login

Both apps have terrible recent reviews on the Apple App Store. They need to fix the login process on the apps.

Update: After disabling Block pop-ups, turning off Hide IP Address, disabling Block All Cookies, and clearing History and Website Data for Safari on iOS, was able to log into both the Lufthansa and M&M apps. It is the only airline app for which I've had to do this, not for any other airline.

However, I cannot save the entry data for Advanced Passenger Info System (APIS) on the website or via the app. Filling it out and saving results in errors.