Office 365 login prompt/ window keeps popping up in Outlook 2016

Hi BerndS88,

This issue could be caused by numerous reasons. Below is a list of the most common reasons for this problem:

• Outlook is configured to prompt you for credentials
• Incorrect password cached in credential storage
• Required Authentication Settings for outgoing server and incoming server
• Outlook Anywhere is not configured to use NTLM Authentication
• Corrupt Outlook profile
• Slow or unstable network connection
• Antivirus programs
• Shared calendars

Please check the following points:

Outlook is configured to prompt you for credentials

• If you are using an Exchange account, there is a setting in your Outlook profile to always prompt for logon credentials. To find the setting, please follow:
• 1. In Outlook 213, go to File tab > Info > Account Settings > Account Settings…
• 2. Highlight your Exchange account and click Change button.
• 3. Click on More Settings button.
• 4. Select the Security tab.
• 5. Under the User Identification, make sure the “Always prompt for logon credentials” option is not selected.

Incorrect credential cached in Credential Manager

• Your Windows system stores user credentials on your computer. If the cached credentials are incorrect, you will be prompted for password. This issue usually happens after the password for your domain user account is changed. Before your password changed, you saved your credentials. The stored credentials are not overwritten when Outlook triggers the authentication prompt even if you enable the Remember my credentials option.

To fix this issue, you need to remove the corrupted credentials, follow these steps:

• 1. Click Start, click Control Panel, and then click Credential Manager.
  
  Note If View by is set to Category, click User Accounts first, and then click Credential Manager.
• 2. Locate  the set of credentials that has Outlook in the name.
• 3. Click the name to expand the set of credentials, and then click Remove from Vault.
• Repeat step 3 for any additional sets of credentials that have the word Outlook in the name.

Slow or unstable network connection

• If your network connection is slow or unstable, Outlook may lose connection to the mail server. When Outlook try to reconnect, it will prompt for the credentials. To verify this, please switch to a stable network environment and then check if this issue persists.

Corrupt Outlook Profile

• Corrupt profile may cause Outlook to no longer remember your password. Please try to create a new mail profile in Control Panel and reconfigure your email account in the new profile to fix the issue. To create a new mail profile, you can refer:
• http://support.microsoft.com/kb/829918

Antivirus software

• Antivirus software usually integrates into your Outlook and it may cause some conflicts. Try to temporarily turn off your antivirus software to check if the issue will be gone.

Thanks,
Mouran

Dear Mouran,

Thank you so much for your detailed list of possible root causes. 

I didn't have time yet but it seems that I found the reason for the problem. It was a "Shared calendar" from a client Sharepoint that I connected to my Outlook using another Outlook account (created by my client's IT department). 

Thank god I found it. Since I just deleted it, I can just assume that this should've been the problem. If it wasn't I'll get back to you. 

Also I'd like to apologize for my long response time. I'm quite busy at work and I wasn't able to go through your list and answer quicker.

Again, thank you all very very much for your help!

Best regards,

Bernd