Look first tried to upgrade via SEUS. Maybe he will fix your problem. If not, then you have 2 options:

1. If you have your phone under warranty , take it to service center.
2. If not, the make a full flash ( main, filesystem and custom file )

You can find all files in download section.

Here you have :

Step by step instructions for flashing a phone using only XS++

Flash your phone using SETool 2 Lite

Custom files - description

Languages

Nice day