Hi Maxim,
I have some good news!
The popup didn’t come when all add-ins except the Axigen one are disabled.
So, I did some more testing and found that an add-in named “Outlook Change Notifier” is causing the problem. This add-in is made by Apple and I am guessing it gets installed when iCloud is being installed. After removing this add-in, the issue has disappeared.
Hope this will help other customers running into the same problem.
Thank you for helping me to sort this out!