What WOPI API needs to be implemented in order to support createDocument OfficeJs API calls

Question

I am developing a Word Online add-in that is supposed to operate in an environment with a custom WOPI host. We faced issues with a certain Office JS API which appears not to be supported. But we are struggling with identifying what exactly is missing.

Could anyone please help me to figure out what WOPI API needs to be implemented in order to support createDocument OfficeJs API calls?

For createDocument we supposed to receive PutRelativeFile, however, there was a client-side call to https://word-edit.officeapps.live.com/we/CreateNewDocument.aspx which resulted in a redirect to the Outage.html. Nothing seemed to come to the WOPI host. We already verified that SupportsUpdate is set to true and UserCanNotWriteRelative is set to false.

Word.run(function (context) {
    var myNewDoc = context.application.createDocument(base64String);
    context.load(myNewDoc);

    return context.sync()
        .then(function () {
            myNewDoc.open();
            context.sync();
        })

Is there any configuration to allow this apart from mentioned above?

Any ideas are highly appreciated!