I've seen a few topics now here with OnlyOffice settings in question, but I have it working with this one minor annoyance that I can't get rid of.
OnlyOffice is setup and working on my server, and I can view the files in the MSOffice Web viewer without issue so there is nothing wrong with the file.
I setup OnlyOffice to be the default viewer for Word files in the admin from Web viewer, and it loads promptly but immediately displays a Warning popup:
The thing is, I can SEE the word file loaded behind this warning. If I click okay, then "Close Menu" I can get back to said word file. Everything works from this point. Consulting OnlyOffice TS&R guide here, 3rd item down, it describes a few issues that generate this error message.
I'll be frank, I'm not a programmer. I couldn't program my way out of a paperbag in a snowstorm in Barrow. So I don't understand what it's specifically asking for. Doing the due diligence with F12 in FireFox, I don't see any issues with the load in console:
No blatant smoking guns. X-Frame-Options is properly overridden by CSP (when I initially set this up I did not have CSP headers and so I just got a blank screen with an error for XFO, that at least was an easy fix).
Customer support service by UserEcho