Do you have similar issue at any other wp-admin pages, except User Role Editor?
It really could be the reason of a problem:
https://www.varnish-software.com/blog/step-step-speed-wordpress-varnish-software
Pay attention on the item “2. Add the plugin”. If that will not help, try to check with your server support if Varnish is configured to work with WordPress properly according to the document above, especially this item “9. Exclude URLs”.
