"Untrusted Domain" error when connecting to SQL Server localhost and specifying port
So, we just had this problem, adding details in case it helps someone else and they don't spend time going down the rabbithole of Kerberos debugging like we did. 

I don't have all the details because our next level tier fixed it, but apparently in our case since we had a cross-forest trust authentication, the issue was that we needed to grant the user's group "Allowed to Authenticate" permission on the server object they were trying to connect to, after that it worked fine.

we had a win2k16 server running Sql2017 in domain1, and users running win7 in domain2 with trusts setup between the two, this worked fine.

But when the users migrated to win10, they received the above error. Fix for us was granting 'allowed to authenticate' to the domain user group in domain2 on the server in domain1, then win10 worked fine.

If anyone knows why upgrading client OS makes any difference please let us know!