Cannot add new events on Outlook Calendar

Since a week or two ago I haven't been able to add new events to my Outlook Calendar on my laptop. However, I can add an event on my mobile and this is then visible on the laptop as well.

After I have created the event, the error message is: "Could not save event. Try waiting for a while. Keep event open, so you can try again later. "

I tried updating Office, but it did not help.