since I don’t believe the qbt executable is signed.
Yup you are correct, another reason that anti-virus/malware type software will mess with the download or execution of the installer.
Based on the current info that's kind of my initial hunch. The installer could crash if the user's anti-virus/malware messed with it. We also don't know if there is other software installed on the system doing things like that..
Otherwise, ruling out other things could be just that Windows itself is possibly borked. The sfc / dism method may fix that. Installers definitely crash when something is wrong with the Windows OS.