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.