confirmation: The unexpected behaviour still appears in the recent build 4.0.3.1476 RC.
description in few words: Pasting a picture into the AlbumArt works fine, if the picture is loaded from the filesystem (File Properties -> AlbumArt -> Add...). But MM hangs if the same file is copied into the clipboard and "right-click empty AlbumArt" -> Paste is selected.
Any suggestion, what should I test?
Cheers, M

