It looks like our Paypal integration suffered from an expired API certificate.
Sadly Paypal has changed their website so often, that their own documentation on how to renew it (that includes the scary step of deleting the old one and doesn’t mention that the „renew“ button only exists unless all but one old certificates are removed), is completely out of sync with how their website works nowadays.
Paypal should work again now.
If not, please contact us!