Storekit problems (SKErrorDomain error 0.)

I have implemented in-app-purchases in an app (WordBrain for iOS, a word puzzle game) which has become surprisingly successful.

I live track errors from installs to improve the game.

Too often the call to appstore to get product data fails. That is:

Storekit.requestProducts(identifiers, function (evt) { ... }

evt.success == false

evt.message = "The operation couldn’t be completed. (SKErrorDomain error 0.)"

Why is this happening? What can I do about it?

This is summing up to quite some revenue losses every day now. :-(

Any help here much appreciated!

Thanks, Tobias