Thanks for your reply, Quaksen!
I'm not sure if there can be an antivirus connection; it looks like *maybe* a Windows Defender screen is opening just before the modpack installer fails. However, I've tried disabling Windows Defender and there's no change.
I suspect there may be something cached which is left over after having uninstalled the program, but I wouldn't know how to find it.
Either: 1.) I accidentally force-closed the installer while it was working, and some lingering error or corruption from that, probably a cache file, is preventing the modpack from installing at all now.
Or: 2.) The modpack closed itself mid-installation (or was closed by some antivirus program, which would be weird especially as I got no alerts from any), and continues to not work either for the same reason or for the reason in number 1.).
Reboots, uninstalls, new downloads, disabling AV software - so far, no solution.
Best,
FLOT