The only thing i can think of is that your either using a firewall that blocks port 81, or the proxy server your using doesn't allow websites that sit on port 81 (we use that port as the webserver that runs on it is very lightweight and it helps reduce load on the server heaps).
I'd suggest that you either allow port 81 requests or dont use the proxy server for the 613 sites.
For a test to see what your problem is maybe try accessing this URL:
[
images.plus613.com]
If your redirected to the sites frontpage then firewalling/proxy isn't your problem. If however you do have something blocking hopefully you should get a error page expressing what your issue is.
Let me know how you go.