Should anyone be interested...
I never solved the problem with SMB file sharing. However, I installed NFS on my WHS2011 box and (almost) everything started working properly. No more failures to start playing after I left the box for a few hours. However, there is a character set problem - WHS2011 only supports ANSI (and some Chinese options) and rune audio expects UTF8 or ISO.
I then discovered hanewin (
http://www.hanewin.net/nfs-e.htm). It costs 19Euros, but it appears to work perfectly. Add ",udp" to the advanced mount options for higher performance on the rpi (I get 35Mb/s when rebuilding the library).