Uninstalled the 6.2.1.3 x64 (not fully, only via the uninstaller), downloaded the 6.2.1.5 x86, and interestingly enough it installed fine.
I then uninstalled the x86 (again only by uninstaller) and then tried the same 6.2.1.5 x64 installer again and now it magically works.
Something definitely wonky in the installer code, but I have 6.2.1.5 installed now, so whatever.
rytomi
Glad to hear you got it working. It's a very unconventional way of doing it, but the end result was a good one.
As a curious test, I tried reproducing the same result and it was easily reproduceable. It seems if the offline installer is run when StreamFab is still running, the installer crashes and messes with something that won't let it run again afterwards no matter how much I try even without StreamFab running or installed anymore.
I'd have to put a big part of the blame on the StreamFab developer, in this case. No well written installer would allow you to run the installer while the program is already running. I sometimes have a nasty habit of forgetting to check some minor programs I run to see whether they are running or not when I start their installer for an update. That's mainly because I have their taskbar icons hidden. Well, I immediately get yelled at by the installer. That's the way it should be with SF, as well.