Plasma online accounts or Kontact won't connect to Nextcloud

I cannot log in to my Nextcloud using Plasma online accounts or Kontact calendar or contacts.
When i try to log into the Nextcloud online account (from Plasma settings), I get the error

“Unable to connect to Nextcolud at the given server URL. Please check the server URL.”

I can access the server URL ( with a browser.

When I try to add the Nextcloud account to Korganizer (using this guide … de-kontact), I get all the way to the “test connection” part, and when i do test the connection, I get the error

“ An error occurred: There was a problem with the request. HTTP error (0).”

I get the same error if I try to configure the connection manually.

The really weird part is that I can connect using Gnome’s online account feature, both on Fedora and Garuda.
Additionally, using the same machine and plasma install, I can connect to other nextcloud provider, so I think the problem is with my server.

My docker-compose is here.

The Nextcloud server is 20.0.11, its calendar app is 2.3.1 and contacts app is 4.0.0. Running on Ubuntu 20.04, with docker container setup.
My client machine is running Garuda linux.

I don’t know how to even begin solving this?

Don’t know what it was, but I tried the same with Kubuntu and it just works, so maybe there is something wonky with Garuda/Arch version.