Itunes: Can't Add Music, Etc. (error -50).

[5DIME]

Whenever I try to add music, add a playlist, anything, I keep getting this error:

The iTunes Music Library file cannot be saved. An unknown error occurred (-50).

I've uninstalled and reinstalled iTunes, thrown out preferences, repaired permissions, etc.
No luck.

I'm running 10.3.9, iTunes 4.8 and am pretty savvy.

Help.