I solved this myself, but I wanted to make this thread just in case others experience it, as I didn't find a solution on Google for my exact problem.
I've been implementing Prime31's Google Play In-App-Billing on a new unreleased game. The in-app-purchases used to work fine in my test build, but then I started getting the following error message when attempting to purchase an in-app-purchase:
Error retrieving information from server. [RPC:S-5:AEC-0]
This was shown on the "Play Store" purchase screen. I tried to reset my phone, delete my account on my phone, clear Play Store apps cache like a lot of people suggested when I Googled this, but none of it worked.
However, the problem was that I had just uploaded a new build to the Google Play developer console. It seems like you have to wait 1+ hour for it to get properly updated for new build versions. After waiting, it worked fine. But I could have saved a lot of time and problem solving if I had known that!
This must be something that only happens on the newer Play Store versions, as I've implemented In-App-Purchases before, and uploaded a new build & tested it right after, and it worked fine on those.