It may be trying to download the patch, and it gets corrupted because the path its trying to save to is too long. Its not a windows Problem. Its an Unreal still uses Ansi version of CreateFile() which has a limit of 260 - (8.3). = 248 character limit on the path.
Do you know what the path is? Mine wasn’t very large. “d:\dev\UE4Versions\4.19p\UnrealEngine-staging-4.19.” ~ 51 character to start, and this fails to patch.
Moving it to "d:\dev\UE4Versions\4.19p\UE4" ~ 29 Characters to start, allowed the patch to succeed.
If you are installing to "C:\ProgramFiles(x86)\Epic Games\4.18" ~ 36 Characters…
I dont know what the path length is of the downloaded patch, but it might be worth trying to install to a directory that is shorter… "C:\Unreal" Or something short like that… And see if it will succeed.
That’s the only issue I’m aware of at this time. And its a problem with both the 4.18 and 4.19 versions.