As it turned out today, the cause was incorrect software configuration by other server administrators, on the server where HeroesWorld is hosted, which made the *.heroesworld.ru domains inaccessible outside of the server.
The problem was discovered and the settings were restored on the evening of November 30th. Website, forum, and chat monitoring has also been added, so a similar problem is unlikely to occur in the future, unless HeroesWorld and its monitoring system fail simultaneously.