Some of our clients are running mod_security on their boxes (which isn't a bad idea) and recently some of these clients have begun having issues with the visual integration working for J!WHMCS Integrator. There is a simple fix that is already in the software to resolve this issue that I wanted to pass along to help others that may be encountering this issue.
How to know if this affects you
If you know you are running mod_security and your J!WHMCS Integrator visual integration suddenly stopped running for no apparent reason (and everything checks out in the backend of WHMCS > Addons > JWHMCS > System Check) this may be you. To confirm, go into WHMCS > Addons > JWHMCS > Settings and enable the debug option and hit save. Then go to the front end of your site (index page) and append ?m=jwhmcs to the URL. So a typical URL would look like:
Then click on the tab that says "Check Rendering". You should now see an 'http_code'. If it indicates '406' then this fix will more than likely resolve your issue.
How to fix the issue
Go into the backend of WHMCS and go to Addons > JWHMCS > Settings > Advanced Settings and enable the option to Pass User Agent. Then click Save and refresh the front end of your site. Your site should start wrapping at this point if the mod_security issue was causing the problem.