Can no longer edit documents in OnlyOffice online

I set up OnlyOffice a month or two ago, and it used to work. Now when I try to edit a file, “test.docx” it loads a blank page with the following message:

The community document server only supports up to 20 concurrent sessions

The odd thing is I only have one session open. No other users are editing documents.

Community Document Server 0.1.5
Nextcloud 18.0.3
Ubuntu server 16.04

I had some probblems with document server App. Try to install the OO dev on a subdomain.