Office 365 Integration: Real Time Notifications Error

[mgb]

Hello Forum

I get the following message during integration:

Real Time Notifications to Office 365 changes will not work because it seems that HTTPS port forwarding is not configured. This means that when changes occur, they will not be updated real time. A synchronization will still occur every night.
Code: InvalidRequest Message: Subscription validation request timed out. Inner error: AdditionalData: date: 2020-09-11T08:30:18 request-id: 909b23c7-00fb-482f-82ce-bf50684b820b client-request-id: 909b23c7-00fb-482f-82ce-bf50684b820b ClientRequestId: 909b23c7-00fb-482f-82ce-bf50684b820b

The firewall check says that everything is OK.
What else could it be?

Greetings