GIT Fatal error: Authentication failed in Visual Studio

To force the Git credential Manager to stop interfering, you can add the following to your global git config:

[credential]
    helper = 

This effectively turns off any credential manager. In Azure DevOps you can then configure a set of Git Credentials or a Personal Access Token and use that to connect directly.

It doesn't explain the problems you're experiencing. Given that your account seems to be Machine Bound and an AAD account a lot of things can be in play. This includes AAD Home-Realm Discovery and Single Sign-on and Azure Active Directory Guest accounts.

Another possible problem is your Windows Credential Store which has multiple locations where Azure DevOps and Git credentials are stored. And finally the account used to sign-in to Visual Studio 2019 to retrieve its license may be playing up.

With the information you've provided it's neigh impossible for us to help you resolve the issues without looking over your shoulder. I'd almost suggest filing an Azure support request if you have them available so an engineer can look with you.

Optionally run

git-credential-manager.exe erase

And pass in the following data

protocol=https
host=dev.azure.com
path=/jessehouwing/vsts-extensions/_git/vsts-github-tasks
{newline}

To tell git to delete the credentials stored for your account

Try and check the value of git config credential.helper.

If you see "manager", open the Windows Credentials Manager, and put as a password the PAT -- the Personal Access Token created -- (while keeping your regular login, associated to the remote URL)

Then try again, and see if the issue persists.