Signup form error for US visitors

Hi,
I’m having an issue with my Klaviyo signup forms. When visitors from the US try to subscribe, the form shows the error “An error occurred when submitting. Please try again later.”

I tested with a VPN in the US, and the error always happens, but the form works fine when I try from Vietnam.

• Double opt-in is disabled.

• Location targeting is not enabled.

• The integration with Shopify is connected.

• The list connected to the form is active and set up correctly.

• I already tried creating a new form, but the same issue occurs.

Can you please help me understand why this is happening and how to fix it?

Thanks!