Finally managed to fix it: SQL Server service account needs "Write public information" permission set to Allow. All tested and working after changing permission.

Thanks for help.

Tried everything with no luck at all. I'll create tomorrow a brand new service account for SQL Service and check everything again. I also tried KERBEROS tool on another server...

Thanks a lot - I didn't know about that tool.

After installing and running Kerberos Configuration Manager I get an error message: Unable to access User Principal information from...