designer/missingid error when importing application definition in ICD 7.6.1 using port forwarding

Background

I encountered a strange error with my IBM Control Desk development environment and wanted to share because I couldn't find any hits on Google.

Problem

The problem I've encountered is the following error when trying to import any application XML into ICD 7.6.1.1:

And the import fails. It does this on all Windows and Linux browsers that I tried (Chrome, IE, Firefox, Edge, new Edge, Chromium, etc.), so it doesn't appear to be a browser problem.

Workaround

I realized that my ICD VM is running on my VMWare host on a NAT network, and I've got port forwarding configured to allow me to access it from systems on my home network. So I figured I would try the same steps using a browser either running on the ICD VM itself or on another VM on the same NAT network. And that works! So the issue is something to do with my configuration of port forwarding or something else in my network AFAIK. More importantly, it's NOT a problem with ICD.

I've used my browser's Developer Tools to analyze what's going on, and I don't see any difference between a failing browser and a working one, so I think there's something going on in ICD. However, I know that this particular configuration isn't one that I'm likely to see at any customer site, especially since I'm using VMWare Workstation to run my VMs. So I'm just documenting this here so there's at least one Google hit on the problem, along with at least one workaround.