As we've said in other places on the forums, we've reported the problem to Apple and they've yet to offer a solution. We have no idea why it's happening, nor why it works for some people and not for others. I'm running Yosemite on a laptop and El Capitan on my development Mac and AppleScript is working fine on both of them.
Make sure you don't have more than one copy of the IndigoServer app anywhere on any mounted disk - that will definitely trigger the error. This is a OS issue of some kind - we use the exact same AppleScript code we've used for many years and since the problem is sporadic it seems to mean that there's something odd going on with the OS.
We recommend that you convert your AppleScripts to Python (as much as possible, that is) since we're not extending AppleScript any more and Python is our scripting language of the future. We know there are still things you can't do in Python that you can in AppleScript, but over time those will go away.