Can't create a new event from thunderbird

[allo-]

I created a new calendar at https://server/user@domain/calendar.ics/ and then created one event. When I try to create a second one, I get the error message:

[2022-03-29 23:43:04 +0200] [22028/Thread-6] [WARNING] Bad PUT request on '/user@domain/calendar.ics/': Multiple VEVENT components with different UIDs in object: '4ff95007-abd1-4def-bd0e-fdb1e8b43d54', 'e2cea9c7-052a-486b-b34e-fed1fc94d9b0'

[allo-]

Thunderbird seems to have created a file calendar.ics on the server instead of a folder, even when I had the trailing slash. Deleting it and trying to request the URL from the browser results in

The requested resource could not be found.

and doesn't create anything at all.