...
As can be seen from the lists, several other URLs also need whitelisting, such as CRLs and OCSPs.
Secunia CSI
This tool monitors installed software for vulnerabilities. We have the corporate version CSI 6. This is a service that registers itself at Secunia to download patch lists, upload results, etc.
Unfortunately the host that is used for communication (agent.csi6.secunia.com)is only reachable via IPv4.
But when configured to use the HTTP proxy everything works as expected and CSI is able to phone home again 
.
Access from IPv4 networks
...