Help! Unauthorized(unauthorized_client)

My subsite is not sending any mails, form notifications (using GravityFroms) or even The Change WordPress Admin Mail.

In my Post SMTP log it shows the error:Unauthorized(unauthorized_client) by all mail attempts (both Gravityform submission aswell as the change WordPress admin mail,)

What does this mean and how do I fix it?

I have a multisite, and on all my other subfolder site it works, just this oneis not sending any mails. All other sites are showing no error in Post SMTP just ‘sent’ instead of the error ‘Unauthorized(unauthorized_client)’