Step 1 - Go to the "Web" section and click "Error page" icon.
(see screenshot)

Step 2 - Enter the new error page file which you want to use.
(see screenshot)

IMPORTANT: You must first disable the Front Page extensions in order the custom error page to work!
Please make sure that you are viewing the screenshots in their
original sizes as there is possibility that your browser is resizing
the images!!!

If you have more question regards this matter, please contact our
support team