system's interface has stopped

Hello,

I have a nova pro 7.8.

After a few months using it without issue, I suddeny have now a problem: I have a continuous popup on screem with message "System's interface has stopped" with a OK button, then a second " The process com.android .phone has beeen interrupted" and a OK button. These popups come again and again infinitly even after a shutdown and restart. This makes the device not useable.

How to fix this issue?

Thanks