Seahub_settings.py looks fine to me.
This might be pretty far out but OnlyOffice Tech Support suggested I try:
/ds-vpath/web-apps/apps/api/documents/api.js
instead of
/web-apps/apps/api/documents/api.js
No explanation was given but maybe you can give it a try.
Another thing, if DNS is an issue with your setup, like maybe you’re running a test server on a captive network or perhaps there’s a firewall issue, you can spin-up the oods container with a specified DNS server.
Default (uses Google 8.8.8.8):
docker run -dit -p 88:80 --restart always --name oods onlyoffice/documentserver
Specifying DNS (e.g. 80.80.80.80):
docker run -dit -p 88:80 --dns=80.80.80.80 --restart always --name oods onlyoffice/documentserver
More on configuring Docker containers can be found here.