Google Play : An error occurred while getting the status of your purchase I'm unable to buy apps from the Play Store. 

This is the message I get when trying to buy the app from a desktop computer :

(Click image to enlarge)



This is what I get when I try to purchase the app from my device (Nexus 5) :
It's in French, in English : 


  Unable to process your order. please try again




I have already tried clearing the cache for the Play Store app on my device, cleared the cache on my browser on my desktop computer, and I tried 2 different credit cards (one Visa, one Mastercard, both working on Amazon).

I am able to download free apps, and was able to purchase apps from the Play Store in the past.
My phone runs Android 5.1.1 and is not rooted.

Does anyone know what the problem could be or even where I could find more helpful error messages?
Silly me, I was trying to purchase the app with the same Google account as the one I use for publishing apps, which is clearly forbidden by Google's policies : 


  Please note that it is against Google’s policies to purchase your own
  application. You will receive an error message when you try to
  purchase your own application.


Pitty that the error message wasn't a bit more helpful, but the mistake was mine anyway.