Attempting to generate PDFs on private wikis (for example, https://otrs-wiki.wikimedia.org) does not work, because cookie forwarding is not (yet) supported by the bundler or by the Collection extension.
I investigated. It appears the VE is using the Parsoid API, but using the cookie-forwarding provided by VRS. See:
So OCG would have to add support for similar cookie fowarding, presumably by passing the cookies as part of the metabook JSON.