Who Me Too'd this topic

I have multiple servers behind a clientless Global Protect VPN Portal and they all work fine apart form the Demisto (Cortex XSOAR) HTTPS, which shows blank page when browsing to it.

The Demisto server works fine when browsing to it directly  and all other sites work ok over the Clientless portal . The GP Clientless content package is up to date and I have been testing with security policy allowing any service/application to the Demisto server. I see permitted traffic in logs with packets flowing in both directions. It looks like that the issue is related to the Clientless Content re-write and I followed the content rewrite troubleshooting page, but had I could not find the exact reason for the behaviour and how to fix it.  I also could not find anything related to the issue in the Demisto support section.