e503 no service

Pound is a popular load balancer / SSL terminator. As it usually happens with open source programs, there is plenty of documentation and self-help available on the internet, but there's one error message I could not find. Pound denied access to the web site and logged the following message:

pound: (756000001700) e503 no service "GET / HTTP/1.1" from 12.34.56.78 www.thedomain.com

I've found out that access to a service has been blocked by a HeadDeny directive. The specific problem was denying requests containing the X-Forwarded-For header - which blocked access by users behind proxies. Changing HeadDeny in service to a HeadRemove in listener fixed the problem.