Communication with the API failed, is NPM running correctly?

[Marco3G]

Hi everyone
I'm trying to set up a new subdomain and get a LetsEncrypt certificate for it. When I do "Test Server Reachability", the subject message pops up.

I have no idea if it is running correctly. The other domains are shown as working correctly and I can add reverse proxy entries. Only SSL seems to have issues.

How do I troubleshoot this?

[Marco3G]

Restarting the container didn't help but restarting the docker host did.