One of SharePoint Farm that I have configured for one client has recently failed with all error shows something like below:
The login is from an untrusted domain and cannot be used with Windows authentication.
After many hours investigating, we found out that this SQL Server had 2 DNS name. This system has been used for many months without issue under this 2 DNS. After AD was upgraded, this error started to appear. Many article pointed to Kerberos issue, but we did not use Kerberos authentication. After removing one of the DNS address, the issue seems to be fixed.
I suspect when the AD was upgraded, somehow the priority of the DNS name might be changed which caused all login to fail.