Posts from The internet connection drops when entering an online game in Heroes 3-5.

Basically, I wanted to play Heroes 5 online with a friend, but as soon as I enter the multiplayer mode, my internet connection drops. The same thing happens in Heroes 3. I have a 3G internet connection from Beeline, and I have no idea how they could be conflicting with each other.

Beeline has poor modem internet, while Megafon's is decent. But it's even better to avoid using a modem if possible. I'm saying this because even if the client doesn't crash, it's unlikely you'll be able to play Heroes of Might and Magic V. And even with Heroes of Might and Magic III, it's not guaranteed.
Regarding this specific error, I don't know, but these types of programs are generally buggy and unreliable. I would, of course, check your connection settings and whether internet access is allowed for these games, but I'm not sure if that will help.

It’s better to use Wi-Fi because it’s even cheaper, and besides, I was able to run a local network through Hamachi on it, but not on a modem.

Basically, I noticed that the game crashes specifically when playing through Hamachi or Tungle. When you simply enter the multiplayer mode, everything works fine. However, without Hamachi or similar programs, the game can't seem to detect other players. It seems like the issue might be related to the encryption of network data.

You can play on ubi.com. But in general, yes, Hamachi and mobile internet are a volatile combination. There's also something called Comodo, which is also for local networks. I played a couple of games on it once; sometimes it's more stable.

There was a bug in Heroes 3, but it was fixed, while in the 5th game, the internet connection still drops.

Basically, the same problem occurs – the internet connection drops during gameplay in Heroes 5, and not just in that game, but in other games as well. The network connection is interrupted when using Hamachi or Tunngle, downloads fail to load, and videos on YouTube and music on VKontakte don't load, and Skype doesn't work either. In general, it's as if the game is consuming all the internet resources.