I found my save files
.
I don't know what happened and it's possible that this is a problem on Linux with proton but I will try to describe it if anybody has the same problem
go to .steam/steamapps/compatdata/261550/pfx/drive_c/users/steamuser
The save files are normaly in My Documents/Mount and Blade II Bannerlord/Game Saves
but I found them under My Documents BACKUP/Mount and Blade ...
So you can just move them from the backup directory to the normal directory.
I diddn't created this backup directory but I found my save files so I'm happy