I was experimenting with Fedora recently and saw that one a lot. And one perfectly ordinary package would simply fail to install after it checked (and listed) all of the mirrors and failed to find it. (Other packages work fine.)
I was using a cacher (sys-cacher). If I shut it off that one package would install, but I’d still get the complaint you describe.
Anyhow, I’m just grasping at straws here, but if you are using a cacher, try disabling it.
That depends greatly on which one you’re using! The one I use has a policy entry in dom0 AND also has some edits in certain config files in the template that is trying to do the update (not the cacher, but instead the one(s) you’re having trouble updationg).
First things first, you are using one, for certain?
Ok, how exactly that post helped you to resolve your issue? @BEBF738VD just explained in it that you can still use qubes.UpdatesProxy, but how to use it he explained in his post I quoted? Now I’m confused how you resolved your issue actually.