Hi, Kolaente, nice to see your reply!
I actually find the reason for that being an unexpected entry in the caldav from Vikunja. There is an additional entry called “projects” (not created by me) showing up at the caldav list that could be added to synchronization. The problem disappears when I unselect it so I think the bug would be why projects is identified as an source of caldav.
Since what I’m having is just a fresh install of Vikunja from docker latest image, I think this should be easily reproducible on any newly installed version of Vikunja.
BTW: I suggest you add the community into the social section of the vikunja main website. It is actually not easy to navigate back to here if I don’t explicitly pay attention to the domain.