Putty error. Unable to open connection to hostname. Host does not exist
I ran into this problem and found nothing via search so I'm sharing the "fix".
If you pin putty portable to the start menu on windows 10, then use the "recent sessions" or "pinned sessions" that can cause this error if the saved session doesn't exist in the registry.
I originally thought it was a conflict when you have the non-portable version installed on your PC, but that's not correct. I uninstalled my local version and it had no impact.