NextCloud 18.01 here on Ubuntu.
I installed OnlyOffice a month ago and it’s been great. I use it every day to collaborate with people at work.
Then, today, with no changes to the server, I found that you cannot open any document with OnlyOffice now, if you are offsite. They download only.
I can still use OnlyOffice onsite. Again, there have been no configuration changes in weeks. I tried rebooting the NextCloud server but it had no effect.