Cannot install Basecamp "Key not valid for use in specified state"

Download the installer fresh one more time. Try that again. As with any internet download, there can sometimes be corruption. If a fresh download doesn't help, let me know.

Nope, it's not corrupt. It even happens if I now try to install anything else from garmin.

Not installable anymore:
Garmin Express (though the installation of Express runs fine, and if I close the installer with task manager before clicking on the error button than it is installed fine).
Garmin Web Updater
Garmin Map Install
Garmin USB Drivers from extracted Basecamp 4.3.5,

Does Install without problems: Garmin Mapsource 6.16 or 6.13
any other .msi installers I tried out.

I am looking into this. I am sorry for the trouble this is causing.

FYI I plugged "Key not valid for use in specified state" into a search engine and got lots of hits. It seems to be some sort of certificate/crypto issue.

One hit I came across said this fixed the issue

• Moved the contents of "C:\Users\%USERNAME%\AppData\Roaming\Microsoft\Crypto\RSA" to another location.
  
• Ran the install again
  

NTE_BAD_KEY_STATE0x8009000B Key not valid for use in specified state
msdn.microsoft.com/.../dd542646(v=vs.85).aspx

'NTE_BAD_KEY_STATE' Repair Tool
aeoid.com/.../

'NTE_BAD_KEY_STATE' Repair Tool
aeoid.com/.../

No offense but that site purports to be a Microsoft support page. It is not. I'd be very wary of downloading and running the so called repair tool.