Hi Dinos Nevradakis,
Thank you for reaching out to the Microsoft Q&A platform.
Based on provided error message it an authentication issue, here are some steps you can take to troubleshoot and resolve.
Could you please check Active directory policies, user permissions and the Limit number of connections policy.
If you are using Remote Desktop client, please check it is up to date.
Check that you are having the necessary permissions to access the remote system.
Please try changing the group policy as navigating the below:
Navigate to Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Security>disable the policy named "Require user authentication for remote connections by using Network Level Authentication".
I hope this information is helpful. Please feel free to reach out if you have any further questions.
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Thank You.