Nextcloud 19.0.0: "ONLYOFFICE cannot be reached. Please contact admin"

I have a fresh and clean installation of NextCloud 19.0.0 on a ubuntu 18.04 server. I am running it under the domain, and I am using properly signed SSL certificates (however I also had the same issue when using HTTP only).

I can access nextcloud at said address, and I have successfully installed the ONLYOFFICE app as well as the community server (after some minor php adustments regarding timing & memory).

Now on my Mac, using the ONLYOFFICE desktop app I can connect to the cloud (using, and I can successfully sign in. The ONLYOFFICE window will then show the contents of my cloud, however when I try to create a new document, or open an existing document, I get “ONLYOFFICE cannot be reached. Please contact admin”.

In the logs there is no message at all, so I dont know where to start. When googling, I found that a lot of people seem to have this, yet I fail to find a working solution.

How can this be solved?