How to debug onlyoffice's document server?

I have set up a linux server with a nextcloud instance (nginx, mariadb + redis, nothing out of the ordinary). https/ssl is realized via letsencrypt. After installing the Community Document Server and the Onlyoffice from within the nextcloud I can’t “launch” onlyoffice (e.g. creating a new file from the “+” button doesn’t give me the option to create a presentation).

I have tried a few different settings in the Administration section for onlyoffice but so far no luck. Currently, I have the following settings:

When I access the nextcloud from outside my local network, hitting “save” gives “Error when trying to connect (cURL error 60: SSL certificate problem: self signed certificate (see”

I’m a bit at a loss here. What could I do to understand what is going wrong here? Does Onlyoffice produce any logfiles? Any other suggestions, e.g., concerning the addresses? Thanks!