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It's apple's internal servers that are causing this. From what I researched a few hours ago, this happens on SOME testers (apparently not all). And it only happens on test accounts, so production app is safe.

I have asked an Apple using Apple Developer Center, and they replied to me like this:

Hello Bartlomiej Semanczyk,

I’ve review the problem description presented below. This looks like a bug report issue to be investigated by the App Store Server QA engineers. The error string presented in the Stack Overflow page is new to me. However, this issue needs to be investigated by the Server QA team as it’s likely that the server is issuing this error string. Please follow these instructions to install the StoreKit profile and capture a console log, then submit a bug report. I will review the bug report and make sure that it’s forwarded to the Server QA team for investigation. BTW, I just ran an old StoreKit sample ad I’m not seeing this issue. I wonder if the issue only occurs with new in-app purchase identifiers.

and here they attached a lot of instructions ho to install StoreKit and capture the device console log.