In summary, the problem is, on my Win7 PC, Steam thinks it is downloading and installing KSP, but all it downloads and installs is a folder containing a tiny .txt file. When I click PLAY all I get is a popup saying missing executable: KSP.app.
In more detail...
I used to play KSP on Steam, but eventually I uninstalled it. Yesterday I re-installed KSP in Steam. As usual, I click INSTALL. The install window says: space required 2456mb, space available 237013mb. Estimated download time 1 hour 15 min. The location for install is my usual Steam Library folder. When I confirm to start the installation, the installing window opens, then closes after just 1 second and Steam says 'Steam has finished downloading KSP'. The INSTALL button changes to PLAY. I click PLAY and it says:
Steam Error - missing executable - D:[folder pathway of my steam library]\Kerbal Space Program\KSP.app
My Steam library says the 'size on disk' of KSP is 171 bytes. All my other games in there are much bigger.
If I go to KSP properties and click 'browse local files', as expected it opens the KSP folder in my usual Steam Library folder (where all my other games are) and the folder contains only 1 file called SteamDep.txt.
If I go to KSP properties > local files > and click 'verify integrity of local game files... ', it says 'Validating files 1 of 1', then after 1 second says 'All files successfully validated'.
I tried uninstalling and re-installing some other Steam games, and they install and play just fine. Only KSP has this problem.
Anybody able to tell me why? Thanks.