Do these symbolic links exist? If not please try another reinstallation:
I saw you already use Letsencrypt cert and FQDN. Then you may just try to update to a newer documentserver (tested until 5.1.1) version. I saw there’s a new version 5.1.2 available, I have to test it. Seems onlyoffice releases quite often.
Thank you very much for this motivating feedback!
This error usually occurs only once because documentserver depends on nginx and default nginx wants to listen on port 80 where apache already listens. With nethserver-nginx, which is required by nethserver-onlyoffice, this error gets fixed.
As you wrote that the documentserver is running (I assume you can see the documentserver page on port 8082) we can ignore this nginx error.
This seems to be an old log. Newest entry is from Apr 17 14:15:21.