[24.04-2.x.] nextcloud sync fails

Since a few days calendar app fails to sync on my OP5T with 24.04-2.x.

I get a popup message that the account can't be synchronised. I am asked if I want to log in again. When I press yes, I get a page where I can choose my account, but I get a failure there too.

In the log there are those entries:

[16.01.26 17:03] Fail to authenticate: "Authentication method is not known."
[16.01.26 17:03] Account with empty sources!: 403

Removing and adding the account again does not fix it.