Self-hosted SN on Synology, Docker: Can't sign-up, log-in (Failed to Fetch, Load failed)

[Fulltimehuman]

I have just installed SN on Docker on my Synology NAS running DSM 7.2 (latest SN version, just pulled). The process went well and I installed a reverse proxy and certificate and can thus access the infrastructure on "https://stdnotes.mydomain.de". I get the message "Welcome to Standard Notes infrastructure! ..." Obviously, port 3000 is open on the router (Ubiquiti).

However, when I try to create a new account inside the desktop app, I am always getting one of the two error messages: "Failed to fetch" or "Load failed" when I enter https://stdnotes.mydomain.de as the custom server.

I do seem to get a connection when using http://stdnotes.mydomain.de:3000, but as soon as I log in, I am being presented with constant requests to login again and a permanent error message "Sync failed".

Any idea what would be causing this? I have not idea where to start looking at logs, as I am completely new to SN...