Hi there Timm!
My name is John and I'm a Guru here at Shopify!
Thanks for posting your issue here on our merchant forums. :)
Based on the info provided here I would say that this is a Stripe issue as it's generated when customers are trying to check out with a credit card and refers to an API key (likely the one used to connect your Stripe account to your Shopify store).
I'd advise you to reach out to Stripe support if you haven't already so they can provide some info to hopefully get you back up and running ASAP. :)
Best of luck with everything here!
I have the same problem for three days, no one has the answer.
I have contact shopify and stripe.
Stripe tells me that you have to enter the new APIs in schopify to make the connection between the two.
shopify, replied that it was a stripe!
and when I ask them to enter API keys
no answer .
I thought it was more professional than that