Syncing my calendar with Posteo.de worked for a while, then stopped a week or two ago. The same calendar URL syncs without problems in Thunderbird on Windows. The same URL works without problems in KOrganizer. Other CALDAV calendars sync without problems in that Thunderbird on Tumbleweed. A different Posteo calender works without problems in that Thunderbird on Tumbleweed. Deleting that problematic calendar in Thunderbird and creating a new one doesn't help.
Just that combination of Thunderbird on Tumbleweed and that one Posteo calendar won't work. Any ideas how to fix that?

TIA