This is a known issue with Wordpress that happens when you pass the variables from your webform to your thank you page.
To fix this, here's what you need to do
STEP 1: Go to Your Hosting Account
STEP 2: Find the File "wp_config.php"
You can find this on the folder where you have installed this Wordpress website.
STEP 3: Edit and Add This Code
Open up this file and add this line of code to the bottom of your wp_config.php file
STEP 4: Save it!
That's all you need to do and it will now work :)