Error trying to mine with stratum_proxy
The problem was with windows firewall. Turned it off, tried again, it worked. Turned it back on and set the appropriate windows firewall exceptions for stratum_proxy and cudaminer and it worked.

I had this problem suddenly appear again this morning. Turning off firewall made no difference. I changed cudaminer to point to my machine ip address instead of 127.0.0.1 and then it started working.