Synthesia Talk

Hello there,

I love Synthesia and I've already purchase it for PC and Mac. I'd love to use it on my iPad, but I just can't unlock the full version.
Every time I tap "Unlock now for only 8,99€", I receive the error message:

Your purchase could not be completed
For assistance, contact iTunes Support at http://www.apple.com/support/itunes/ww/

And then the window shows:

There was a problem purchasing or restoring.
(Leave this screen and return to try again.)

I've already called the Apple support two times and talked to them for over an hour.

Did someone have a similar issue? Is there maybe a problem with the app?

I'm using an iPad 9,7 (2018) with iOS 11.4.
Kind regards

There is a little more information about this problem, here.

Oh yeah, I found that post shortly after posting this.

I hit the purchase button for more than 10 minutes repeatedly and suddenly it worked!