Error “Locally modified game files detected. Play is disabled” in Dota 2, what to do?

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    I encountered a problem in Dota 2: an error appears saying “Locally modified game files detected. Play is disabled.” This prevents me from playing, and I can’t figure out what exactly caused this issue. I haven’t made any changes to the game files, so I don’t understand the reason. I need help to sort it out and start playing again.