on my end the following fixed it:
shell into the onlyoffice docker container by:
univention-app shell onlyoffice-ds
then edit /etc/onlyoffice/documentserver/local.json to include the following under:
{
“services”: {
“CoAuthoring”: {
"request-filtering-agent": {
"allowPrivateIPAddress": true,
"allowMetaIPAddress": false
},
nano is conveniently available in the onlyoffice-ds docker image.
for a good measure I restart docker container after exiting the shell
with docker restart <containerid>