Routing is probably getting confused since more than likely dmz is allowed to surf via http proxy also. But the error in the logs would indicate that the proxy is blocking it which doesn't make sense. Do you have packet filter rule allow LAN-->http to dmz by any chance?
If you do, as a work around, try putting 172.16.1.2 in transparent skiplist under http/s-->advanced-->Transparent mode skiplist. Also uncheck the box Allow HTTP traffic for listed hosts/nets after you add the IP. I am assuming you are using transparent proxy and see if it works.
It still might be a bug though in http proxy. HTTP proxy was fine in 7.450 but is not the stablest thing in 7.460 in my opinion.