Mac App Store Won't Update Apps? This Might Help. 

For the last couple of weeks, every time I got an update from the Mac App Store (for a non-system application), I’d go to update it, and the Mac App Store would ask for my password, think, and then fail with “There was an error in the App Store. Please try again later (20)”.

No amount of googling lead me to an answer, but it did lead me to some nifty debugging. For instance, apparently the App Store has a debug menu. I turned that on, turned up logging, and then watched my console log. It was complaining about the application receipt (the proof that you purchased the application).

So, back to the google to try to see if anyone ever found that error. I thought “maybe the App Store can’t read the place where I have receipts.” I tried to figure out where that is, which lead me to this post talking about how the App Store keeps receipts around in memory.

Well, I figured, I could give it a shot …

killall -KILL storeagent

and voilà, my App Store could update again.

(Which also means a reboot would have worked, but I go out of my way to not reboot my machine, if I can avoid it.)