EDIT: I solved this myself, the issue turned out to be my own fault! I've answered it below.
If I load my save game (sandbox) in 1.4.3 and click on either the Spaceplane Hangar, Tracking Station or VAB, nothing happens. If I click on the Runway or Launchpad I get the windows to select a craft, but nothing happens when I click Launch. Additionally, the Quit To Main Menu button on the pause menu has become completely unresponsive. I also tried loading multiple quicksaves within 1.4.3, but the same issue is present.
After clicking the above buildings/button, the following error is output to the debug window: "Exception: ArgumentOutOfRangeException: Argument is out of range. Parameter name: index"
I can fix the issue if I revert my install to 1.4.1 using the Steam beta options; the save game functions normally in 1.4.1.
I have linked two log files below, the first is from 1.4.3 after clicking a few of the buildings, and the second is 1.4.1 after successfully loading into the Tracking Station.
I'm only using the KIS and KAS addons. Obviously I'd like to remain current with KSP updates but this issue prevents that, any help you can offer would be great, thanks.
https://www.dropbox.com/s/aflylle6jj4lffx/output_log1.4.3.txt?dl=0
https://www.dropbox.com/s/0o8whmvyhb0xm16/output_log1.4.1.txt?dl=0