Comment 23 for bug 95241

I've found the source of this anomaly: at least in my case was CheckPoint firewall (NGX 6.2) and one of the SmartDefence's rules:

SmartDefence -> WebIntelligence -> HTTP Protocol Inspection -> Header Rejection
The connections containing a string "IST" in User-agent parameter are rejected by the firewall.

Deactivating this rule solves the problem.