Your assumption seems to be correct.
What version of firefly are you using, and what filesystem does your music reside on?
Also, since its a MSS+, whats the OS flavour you are running?
On a NSLU2 f.i. to enable proper unicode support one has to load several kernel modules or use special mount parameters. Im nto familiar with the MSS+, but I guess that some of these steps would be needed to fix the issue.
Other than that, you could of course remane the files 😉