'PayPal Subscribe (Sandbox): Things don’t appear to be working at the moment

Absolutely no settings/code have changed, this all worked 1 week ago (used exact same buyer login to perform test subscription and it worked), now I'm getting following error:

Mystical Paypal error

I have checked the seller account settings and "block non-encrypted payment URLs" setting is turned OFF. (found some google posts suggesting to check this).

Any help greatly appreciated, as you can see PayPal's error is not very helpful in helping to diagnose the problem.



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