Thanks, Jean-Serge. Yes, you are correct: we don't have Content Manager, unfortunately.
As for the urlPermission, we're actually using http only. The https test I mentioned in the original post was just a curiosity on my part, to see if it resolves anything. The base URL that I provided to Support was also http, so I wouldn't be surprised if they hard-coded only http, which is fine. That said, I've asked Support to provide the exact URL that they added to our serverConf.xml file. I'll update once I hear back.
In the meantime, do you have other thoughts as to how to fix this?