The Website Cannot Display Page Error

After submission to my forms many users keep getting the error "The Website cannot display page" error. The form is still getting submitted but the user does not know that so they submit the form multiple times. 

Is there a way to fix this issue?

The issue may have been in your Redirect to external link.

Your thankyou.php may not be working properly.

Kindly check it and let us know how it goes.

Thank you.