I had a few problems with Web Check-In too when we had about 50-60 days for departure: kept erasing my phone number; couldn't register credit car; couldn't upload photo...
Decided to try to do it on MSC for Me app. It said I could only do it 30 days before. Did it 25 days before departure, everything went smoothly (except for the pic - no upload) and I ghave the e-tickets with me 22 days before departure.
Try the app 😉