If you get this error unexpectedly, you may need to manually tell Dapper you want it to directly access your Music library and not export it to a file:
See this article for info on how to set Dapper back to Direct Mode instead of wanting to get to XML files. If this does not work, you may manually have to reset it:
Quit Dapper
Open a terminal (Applications->Utilities->Terminal)
Type: defaults write com.map-pin.Dapper audiophileMode 0
Hit enter and then relaunch Dapper.