Please provide logs of Document Server from /var/log/onlyoffice/documentserver for analysis.
Please share whole log directory as an archive. You can upload it to any cloud storage and share an external link to it (do it via PM in terms of safety).
Thanks for the logs.
I can see that you are using a self-signed certificate or a certificate issued by a local CA.
Please try setting rejectUnauthorized parameter in /etc/onlyoffice/documentserver/default.json of Document Server to false, restarting Document Server services and checking the situation again.
I have contacted you via PM, please see my message.