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I don't see any problem with someone getting frustrated and not wanting to communicate with their opponent anymore (including becoming uncooperative). This is most likely some kind of neurosis that may lead to a more serious problem over time, but it may not, and in general, it's not inherently a major issue.
But one thing is, in my opinion, definitely unacceptable in this situation: if playing with password-protected saves, one of the opponents doesn't share the password with the other after the game ends. This should happen explicitly, and in a specific player's history, it should happen a maximum of 3 times. Twice as warnings. The third time – a permanent account ban. In my personal opinion, not sharing the password (without any discussion or connection to any other issues where different viewpoints might exist) is absolutely unacceptable and rude behavior.
There's nothing to discuss here – the game has ended, so you share the password. If it's possible to automate this to completely remove the "personal" element, so that the server shares the password instead of the opponent, then it would be ideal to automate it. In the past two years, this situation has happened to me twice with different opponents (this was in tournaments; there were also cases in seasonal games). In one case, the opponent at least made it clear that there was something "personal" going on, even though he considered it beneath him to explain what exactly. In the other case, I saw the opponent for the first time; we didn't talk much on ICQ, and in the game, he just appeared out of nowhere, attacked, sacrificed his main hero against my hero and army, immediately left, and disappeared silently. It was completely bizarre.
In my opinion, this situation is directly related to the topic being discussed, because every "cheater" is interested in preventing the opponent from being able to analyze the saves after the game. And if they can unnecessarily sabotage the ending and deprive the opponent of this opportunity, they will do it.