Thank you! I'm surprised about the error.. it sounds like it's using an old .jsp file, I changed that exactly to support people having the same artist in multiple media folders. But 0.5.42 Genres and Related artists works for me.
Could you first of all try cleaning your cache from preventing your browser from using old files? Then check at the about view where it says Version, and confirm that it says "Built with MusicCabinet plugin, version 0.5.42, on May 21, 2012"? If that doesn't work, you could also stop your Subsonic service, and remove any directory below <subsonic dir (probably c:/subsonic)>/jetty/, with a lower version number like 0.5.3x or what you've been using before. It shouldn't pick up anything from there, but just as a last resort. And then start again. Let me know how it works out! It's good to know the cause of every single error that occurs, to help make this better.