Sometimes an In-App purchase fails with the code SKErrorDomain = 0 "Unknown error." in production

Question

I have a Mac app that offers an In-App purchase. From time to time, I have users who will contact me because the purchase ended with an error below.

Please note that this happens to regular users. It works great for me. (There are many questions about SO where this happens for developers with “test” user accounts, this is not the case here).

Error:

Error Domain=SKErrorDomain Code=0 "Unknown Error." UserInfo=0x7fead1e44490 
{NSLocalizedDescription=Unknown Error.}