Hi Jan,
I understand this issue has affected users in your organization, and thanks for reporting the issue with detailed information.
Does the issue also occur to OWA?
I notice that you have contacted our phone support engineer. Have you received a service request (SR) number? If possible, could you please provide the SR number so that I can help you check the progress of the ticket and get more information about the issue.
To protect your privacy, I have sent you a private message. Here are the steps to access the private message:
1. Please go to Your details section on the right side of the community site.
2. Click Private messages.
3. Click the subject title to read the message and reply with your information.
Thanks,
Jason Jiang