The line in the download manager still unfixed (😡), but I found the way how to fix it manually.
Go to C:\Program Files\Maxthon\7.3.1.3000\skin\mx6.7z
If replace file "download_background_dark.png" from "default_200_percent\default" folder to "default_100_percent\default" folder, the line dissappears (browser restart is required).