Outlook not synching with web calendar

I want to (re-)subscribe to a web calendar from icloud.com that stopped synching.
iCloud provides a URL (beginning with webcal://p101-caldav.icloud.com/published/2/...)

that I can enter on the Outlook web client on outlook.office.com (Add calendar > Subscribe from internet > enter URL).
Unfortunately Outlook is not fully synching the calendar any more.

To identify the culprit I used the same link on Google calendar and it could sync without a problem.
So the problem seems to be on the O365 side.

Are there any solutions to this problem?