I get this error every time I try to launch Subsonic on my Mac Mini PPC running 10.5.8 Leopard. I've tried all the updates. I also got the same error when I was running the most recent version of Tiger.
I would have copy pasted it but it wouldn't let me for whatever reason.
Since all I do is pick and choose the songs I want to listen to (and occasionally upload music here and there from my phone), that version works fine for me. It actually runs better (well, faster) on my PPC Mac than on my Win XP or Ubuntu 8.04 boxes (even though they have the more recent versions of the program).
You can use 4.2 on a PPC as long as you wish to use the Standalone install rather than the application. It does require a little knowledge/use of Terminal though.