https://bugs.winehq.org/show_bug.cgi?id=4825
--- Comment #32 from Felix Zielcke fzielcke@z-51.de --- I played now a bit around with a virtual desktop. Winamp Library was shown again with my usual wineprefix.
I think this is related to this bug: https://bugs.winehq.org/show_bug.cgi?id=15346
If you don't move the Winamp windows around there's no problem with the library showing.