Woocommerce JSON parser error

Hi folks,

I’m having a serious problem with the checkout in woo commerce:

After putting in the details (name, address and so on) I want to choose the payment method. unfortunately sometimes I can’t even click into that field. it is in light grey. That means I can’t change the payment method.
If I try to send the form with “jetzt kostenpflichtig bestellen” I get the output:

SyntaxError: JSON Parse error: Unrecognized token ‘<‘.
written before the details. And I can’t finish the order.
Strangely it worked fine until this morning. All test orders the last days went through without problems.

you can see a screenshot here:

You can find the shop of the site here:

Can somebody tell me how to fix it?

And before you ask: the only thing I did today was adding 2 Footer-menues. Nothing else. 😉