Great to know that the issue has already been resolved and thanks for sharing the solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others", I'll repost your solution in case you'd like to "Accept" the answer : )
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Issue Symptom:
Currently our mail.accounts are on an outside cloudbizz server.
So when i add an account to outlook, normally a Windows Popup will ask me to introduce the mailbox Credentials....
Bus since yesterday, i have one account (so far) where i always get a Microsoft Prompt. When i close this prompt, i will sometimes get the Windows prompt, so i am able to configure the mail account as a 2dn account on my outlook.
But from here on, when i open my Outlook, everything is fine and connected for 30 secondes, and then i get the Microsoft Prompt again. With the prompt open, the mailbox stays connected. Only when i close it, it gets deconnected saying "Password required" in the bottom line on the right.
**Resolution:**Adding the account as Exchange 2013 or older and putting the following in the Registry:
Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover and create DWORD key ExcludeExplicitO365Endpoint=1