Add-in issues with Version 2606 (Build 20131.20090) of Classic?

Hi, I’d treat this as a possible add-in compatibility issue with that Office build. A good first test is starting classic Outlook in safe mode with outlook.exe /safe. If the issue disappears, disable the add-ins and bring them back one at a time to find the one that breaks. I’d also check whether the affected add-in vendor has a note about Version 2606 or Current Channel builds. If this started immediately after the update and affects business users, temporarily moving one test machine to a previous working build can help prove whether it is a regression before you roll back broadly.