Hello there,
I'll add my input: I couldn't download Houdini from the main page through Firefox… I tried at least a dozen times, and every time the download stopped at some point (not a precise point). The installers I managed to download where corrupted and could not install due to a NSIS error. I even tried to bypass the integrity check with /ncrc, but the install crashed at some point. This happened with different DL'd versions from the
main DL page [
www.sidefx.com], twice using Firefox to DL (you can restart the download if it fails, FF will take back where it was) and once using a download manager (Free Download Manager). The install simply failed 3 times.
However, I could install the Qt4 version DL'd through the manager from the main DL page, and also the last Qt5 version found in the
production build page [
www.sidefx.com], even though the latter was DL'd through FF and restarted after one failed attempt (could not find the precise URL for the DL manager XD).
TL;DR: My guess, the Qt5 version on the main page is corrupted (the MD5 was different): simply DL through the daily/production build page and you should be fine, even if you have to restart the DL.
I hope it helps!