Redirect problem

My Fast Secure Contact Form plugin has started redirecting users to a standard “thank you” page instead of to the payment page the “submit” button was previously linked to. When I go into Advanced Settings on the form, manually change the redirect settings to the payment page instead of the main page, and press “save changes,” the page resets and none of my changes have been saved. I haven’t installed any new plugins lately that might conflict with the form, but did begin using Google Analytics.

Any suggestions for what might be wrong and how I can fix it?

Thanks!

https://wordpress.org/plugins/si-contact-form/