password not accepted only on my iPad pro 9.7

I can't login to the ITune App Store on my iPad pro 9.7 (model MLT12LL/A iOS 9.3.1) .

The password is correct I use it without problem on line (appleid.apple.com) I login without problem on my iMac, on my iPhone, the password is correct also in the icloud keychain.

I contacted the apple support (Case ID: 1096787407) without success I have been invited to set an appointment at an apple store (Case ID: 1097270465)

the problem at the end seemed solved restoring the last backup.

Now, after 2 days, the problem shows up again.

I can't restore my iPad every 2 days!!!!