Thanks for trying to help me. Unfortunately the same thing happens with Chrome and Edge. So, likely not assoc. with Firefox update. There was a Windows update lately as well... oh well. I have a workaround to use the actual Outlook app instead of Outlook.com calendar. But... soon I'll get a new laptop which I hope will solve the issue.

I allowed a 3rd party cookie and it works for me. see screenshot

Thanks, this looks promising. Unfortunately, I seem ignorant of how to allow the 3rd party cookie. I do not get a message that anything was blocked... welcome a detailed process description for this more ignorant user.

My fault, see screenshots.