SSPI error on windows authenticated remote access only
We finally found how to fix the issue. Unfortunately we do not know why it fixes it as the old VMWare image have the same setting it has on the Hyper-V image. Both suddenly stopped working.

To fix it we had to change the SQL server instance that has been running on the main Domain admin account for the last 10 years to run on LocalSystem and both SPN were registered under a user account. That user was another domain admin account which was the one originally used when SQL was first installed 10 years ago. That account is still active and used. We removed the SPN from the account and put them directly under the SQL dns.

Note that i don't know that interface where SPN are listed but i understood that it list all computer DNS names and all user account of the domain and SPN can be under either. Both IT have no idea why this suddenly stopped working. According to oldest image backup of the servers those settings never changed but that's all we needed to do to make everything work again.