iOS purchases suddenly not going through anymore (error: 21004)

Our LIVE in app purchases are suddenly failing (started yesterday) with the following error:

validateIOSReceipt:iTunes Sandbox validation result: 21004 (InvalidSharedSecret) from https://sandbox.itunes.apple.com/verifyReceipt

when they used to work just fine just two days ago.

We have not changed anything at all in our configuration (especially not the secret) and we are contacting Apple in order to figure out what may cause this but could this also be an issue on PlayFab's side?
We have two projects who rely on the same secret and they are experiencing the same issue.
(They have not changed it either.)

Thanks in advance.
Cheers,
Laurent

Also I must confirm that this is a live/release environment so I am not too sure why the "sandbox" environment is in use here but apparently we have no control over that.

Maybe this is actually the issue?

@Citrus Yan thanks for your reply and sorry for the delay, this was actually caused by an expired secret. Renewing it fixed the issue..