Hi Again,
I've figured out how to fix this for my case, no guarantee this could work in other cases.
DNN was not loading anything, so I figured there must have been something wrong/missing in whatever points DNN to the right "Export Template" page.
Looking inside a different DNN instance (same version) that worked, I found that one relevant line in the ModuleControls table was different:
should actually be:
so I've changed it.
This created an error [something like "'GetSkin' is not a member of 'DotNetNuke.UI.Skins.SkinController'."] in "Admin/Portal/Template.ascx.vb", surprising, because it's the old ControlSrc path, not the new one!
I gave a look in Admin/Portal/ and found that the broken instance had different contents in there (more and different files). So I've renamed "Admin/Portal/" to "Admin/Portal.old" (had to recycle the IIS worker that runs the broken DNN app) and copied the Portal folder from the working instance. Voilà, package seems to be working now.
I don't know why this happened, and I believe I will never find out: the broken instance had lots of problems in the past so it is going to be difficult to find out when and how this issue started.
I hope this helps,
Sergio