Because in the New Horizons Maximus_Functions.c, the void ReinitInterface sets the .IniFile path like this:
Interfaces[INTERFACE_SELECT_STORYLINE].IniFile = "RESOURCE\INI\"+GetCurrentInterfacePath()+"\select_storyline.ini";
But Maelstrom already knows for any Interfaces object, the .Inifile path will always be the default RESOURCE path (specified in start.ini), so it is redundant. If you compare the 2016 version of that same file/function, all the "RESOURCE\INI\" portions were removed for Maelstrom. Keep in mind, that is not the only place in the scripts where this type of problem will occur, so you need to search all the file contents for similar issues.