For no reason at all, when everyone connects to exchange in outlook, a pop-up box appears asking them to enter their credentials?

Dear @yanhaowen

Thank you so much for contacting Microsoft Q&A Forum. 

Based on your descriptions, I would like to provide a detailed explanation regarding the recent issue where Outlook prompted users to re-enter their credentials unexpectedly, and to clarify why the mailbox database newdb02 was mounted on server ex03, despite its lower activation preference. 

1.Outlook credential prompts: 

The widespread credential prompts were triggered by a temporary disruption in Outlook’s ability to connect to the Exchange server. This was not due to incorrect credentials, but rather a service-level issue within the Database Availability Group (DAG). Specifically: 

• The Microsoft Exchange Replication service on server ex03 was down, causing all database copies on that server to report a ServiceDown status. 
• Simultaneously, the copies of newdb02 on ex01 and ex02 entered a DisconnectedAndResynchronizing state, indicating they had lost connection during resynchronization—often due to transient network or infrastructure issues. 
• As a result, Outlook clients lost their authenticated sessions with the Exchange server, prompting credential re-entry attempts. However, since the underlying issue was service unavailability, entering credentials did not resolve the problem until the affected server was restarted. 

2.Why newdb02 mounted on ex03: 

Although ex03 has the lowest activation preference (priority 3), it became the only viable candidate for mounting newdb02 due to the following: 

• The higher-preference copies on ex01 and ex02 were unhealthy and ineligible for activation. 
• After restarting ex03, its copy of newdb02 transitioned to a healthy state, making it the only mountable option. 
• Exchange’s Best Copy Selection (BCS) algorithm prioritizes copy health, queue lengths, and content index status over activation preference. Activation preference is used only as a tiebreaker when multiple healthy copies are available. 

This behavior is by design to ensure high availability and minimize downtime. Once all database copies are healthy again, you can rebalance them using the RedistributeActiveDatabases.ps1 script or by suspending activation on specific servers if needed. 

Besides that, as a forum moderator, I have limitations in my testing environments and cannot access the internal databases to check your tenant directly. Therefore, I highly recommend that you submit a service request to Microsoft Support team for the most efficient help. This will allow our dedicated support engineers who have broader diagnostic tools and permissions to investigate and assist you more effectively. Reference: Get support - Microsoft 365 admin | Microsoft Learn 

Wish you a pleasant day! 

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