Medstar

This sounds like a misconfiguration on your main PC. Error code 0 normally means that the game thinks you (the host) has left the lobby whenever a new user joins you. Since the game thinks a user is joining a now-collapsed lobby, the error code is thrown. Normally, this is due to some connection being blocked on the host computer.

I would start comparing and contrasting the differences between the two computers in terms of their networking since one works while the other doesn't.

Are both PC's on the same Windows version? Do they both have the same antivirus? Are both PC's firewalls set up the same?

Try to find the major differences between the two, that way we can narrow down the issue.

I deeply apologize for my absence. College life has been brutal to me and has had me away from tech support.

If this issue is still persistent, please do the following:

If both of you are on the same network, then ONE of you needs to use a VPN so that both of you have your own public IP address. Basically, the servers are seeing you as the same person (even though you are on different computers/accounts); the VPN solves this.

Change it from No IP Address to the 192 one. If error still occurs, try ticking Send Delay.
 

If that still fails, do one of the following:

A) Set a port number in FirewallPortOverride in your game's options menu; then forward the same port number on your router (both as TCP and UDP)

B) Use Windscribe VPN and set its 10.1.X.X IP address in your game's options menu when you have it turned on. The service is free and you get 10GB/month to use.

Okay, so, if I'm reading this correctly:

• The troublesome games are disk copies
• It is RA3
• You've patched the game to 1.12 and it is throwing an update error
• You are using the C&C:Online launcher and it is throwing a CA Public Key error when hooked

Here are my questions:

• Which 1.12 patch did you install? (like which language did you pick?)
• Do both games work without C&C:Online being hooked? If so, does it indeed display v1.12 in the options menu?
• Are you launching everything/have you tried launching as Admin?

Is your installation of Red Alert 3 a disk copy?