After trying almost everything on this thread and others, continuing to Google, the only thing that worked for me, in the end, was to start Visual Studio as my AD user via command line:
cd C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE
runas /netonly /user:<comp\name.surname> devenv.exe
Original issue: [1]: https://developercommunity.visualstudio.com/content/problem/304224/git-failed-with-a-fatal-errorauthentication-failed.html
My situation is I'm on a personal machine connecting to a company's internal/local devops server (not cloud-based) that uses AD authorization. I had no issue with TFS, but with git could not get the clone to work (Git failed with a fatal error. Authentication failed for [url]) until I did that.