Thank you, @stormi , I'm happy to know that xcp-ng.repo is not touched. These three hosts haven't been rebuilt or restored recently, no hosts have been added, and I did have to make the above modification on each one of them to get them to work. I suppose it's possible I missed the step to remove "mirrors.xcp-ng.org" from xcp-ng.repo during the initial pool build about a year ago and it's just somehow been working with our outbound Internet proxy. In any event, having "mirrors.xcp-ng.org" in our xcp-ng.repo file has definitely caused us problems in the past with our proxy setup.
TGIF/DMCV!