I had same error that drove me crazy for hours, and was down to the uninstaller refusing to remove the old version and so rolled back the installation after failing at this step. In my case the uninstaller can't handle User Folders on Windows like Documents being a redirected folder to a network folder share, and unfortnately the FME installer puts application data files in Documents.

Do you have any Folders like Documents not being actually in the standard C:\\ location?

The uninstaller does not start. The Uninstalling window flashes for a few seconds then the Error one appears.

I checked for relocated user folders, my user had only the OneDrive ones relocated so I unlinked and signed in again to OneDrive to reset them.

There was a user with relocated folders and I deleted the user altogether.

The error still persists. I think this is a bug not a feature

The problem was that the uninstaller was searching for the old installer, couldn't find it but not asking for the path to it.

The uninstaller asked for the path but I didn't have the .msi file anymore and it wasn't on the downloads page .

I filed a problem to support, and they gave me the solution - C168635

They made the old installer file available. Updating still didn't work as it wasn't asking for the file, just throwing out a generic error but uninstalling (with browsing to the downloaded .msi for the old version) and installing separately worked great.

They also suggested I install the new version to a new directory but I don't need multiple versions.

So, thank you FME Support Team! You're great!