IPCOP's Transparent Proxy vs Pidgin.

Someone asked this on one of mailing lists I subscribe, about how his pidgin refused to work after he installed .21 update:

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I'm not 100% sure, but I recently updated my IPCop install to 1.4.21 and it *appears* that since then I cannot access Yahoo Messenger. When I try to connect, it just says "Connecting" forever. Eventually, Kopete will give some kind of error message about failure to connect.

Pidgin has a few more configuration options. So I changed the "Proxy options" from "No proxy" to "HTTP" and used my IPCop's IP address for the Host and 800 for the Port (Got the 800 from the "Proxy Port" on the Proxy page). Now when I try to connect I get the following error:

Could not establish a connection with the server:
Access denied: HTTP proxy server forbids port 5050 tunneling.

Like I said, I'm not sure if it was because of the .21 update but I can't think of any other configuration change I've made that would cause the problem.

I'm fairly certain the problem is with IPCop because if I use the WinXP version of Yahoo's proprietary client, I can get in if I set it for "Firewall, No Proxies" (which forces it to work over HTTP) I can connect.
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Here's my answer:
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I assume you only have those add-ons, and nothing else.

Are you using transparent proxy? Or have you enabled Web Browser Check? If so, please disable both.

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Actually, it's pretty simple cause. Transparent proxy does its job 'transparently' and Pidgin in its current version can't work that way. Don't know if they've fixed this, but at least, this workaround works. :P