Via GOG installer downloads, not their Steam-equivalent. Uninstalls only through Windows control panel.
If you mean like through the Steam client, then no since I'm using GOG. If you mean through the KSP launcher (I don't remember if that's possible) then no, too, since the launcher.exe crashes with the same error.
It's there alright. How can I check whether it's corrupted when I'm not using Steam? Would be great if that's all it is but I doubt it. I uninstalled, re-downloaded and re-installed different versions multiple times over the months.