Thanks for your replies.
I did not know that both sides of the sync file were needed. I'll provide it next time it happens.
I'm FAIRLY certain it has nothing to do with tunnel servers since we only play in LAN ?
It's only the two of us. We are both on gaming laptops. I installed RA2 and YR on both computers the same way - using the TFD DVD, each with their own serial numbers. Then I installed the patch that I mentioned, then I installed CNCNet, updated CNCNet, and that's it. We have a few custom maps, and I copied those over from a shared drive as well as the patch.
So both games should be 100% identical.
My WiFi router is pretty powerful and should be able to handle this, but I do have a lot of WiFi enabled devices, so I might try CAT5 cables.
I do remember having had Reconnection Errors already 22 years ago and 15 years ago when we used to play the original, and back then we were on regular computers on a wired network.
Is it possible that the patch I used has introduced a networking bug? I posted another question about which patch is the best to use (it has one answer that I haven't checked yet).
I'm sure none of us cheat - we have self-made and edited maps in which the oil derrick money is raised to extreme heights, but we have the same version of the map, and we have had the reconnection error on the basic maps that came with the game and CNCNet.
I'll wait for it to happen again and will send the sync files from both computers.
Thanks!