Windows Store glitch out

I have not been able to complete an in-app purchase in the Windows Store for days. At first, I was always getting the dreaded 805a01f4, but after reading every forum in existence, I decided to follow some advice that worked for another guy: I uninstalled and reinstalled 6tag.

Then my Windows Store error became the even more dreadful c101a006. I have checked and double-checking all my personal information on Microsoft.com. Everything is fine. Why can't I make my purchase? Is this happening to anyone else?

(On a T-Mobile Lumia 640 that's still running 8.1.)

EDIT:

By the way, I saw this thread:

https://www.reddit.com/r/windowsphone/comments/3j21qu/805a01f4_error_when_attempting_to_make_a_payment/

But the solution got deleted!