Found some contact info for demonware. maybe @flatsix can email/phone them to see if they will do anything.
https://www.gamesindustry.biz/resources/directory/company/demonware
The server with the issue is cod5-pc.lsg.mmp3.demonware.net the auth server is only connected for a short time during server startup, once authed it disconnects.
Basically the server tries to update its status every so often with the lsg server, and it either 502's (bad gateway) or disconnects, at which point the status update fails and the server prevents new users from connecting.
So far ive not managed to figure out the packet structure/encryption(des/3des it seems) to build a local status cache so it can resend the server status once the lsg server is working again