Dunker
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A clumsy way is to mess with the game files.
Download Total Commander, change the mod's extension from h5u to zip, extract it, in Total Commander, select the extracted mod folders, right-click, click "files" (top left corner), then "change attributes," check "process directory contents" and "change dates/times" and "current," click "OK," repack it into a zip archive, and change the extension back to h5u.
Added 1 hour 8 minutes ago
Changing the date/time helped. The date of the files in the mod was somehow earlier than in the game. Perhaps the mod was released before patch 3.1 for the version of the game from Booki.
Thanks to everyone who helped!