The issue is not related to the conversion request you are sending.
The Document Server does not allow using internal IP addresses by default.In order to avoid this, open /etc/onlyoffice/documentserver/default.json inside the docker container => find the parameter allowPrivateIPAddress and change its value to true
"services": {
"CoAuthoring": {
"request-filtering-agent" : {
"allowPrivateIPAddress": true,
...
}
}
}
After that execute supervisorctl restart all
and check if the issue persists.