When I call requestProducts method of RMStore instance, productsRequest:didReceiveResponse not get called eventually. Error handler is not called either. The request is just keep going and doesn't return any value. This happens only on an iPad iOS 11.2.6. So I cannot perform any purchases on this device cause I always get the "unknown product identifier" error when I create a payment.
Moreover, purchases had worked earlier on this device and suddenly stopped. I have no idea what might have caused it. I've tried to checkout previous commits when purchases had definitely worked, didn't help.
The problem was resolved when I updated the iPad from 11.2.6 iOS to 11.3. This is kind of weird because as I said in the description of the question, purchases had perfectly worked on other devices with the same iOS version. So it might have occurred some bug inside operating system of this device.