Still trying to sort that one out, i've tried to reproduce this without success. I've added accented character to existing stuff, new stuff, copied stuff; it all works. The only thing i've come across to give me any grief are back ticks, key shared with tilda (~). At this point I'm considering going back to scratch drive type system to store cover art in a "proprietary" method so that characters can't create any kind of havoc. It won't help customers using mPad or other MPD clients, but atleast our web interface would be more solid.
Some data points:
(1) This is a relatively new issue and may coincide with your change to how you are storing art work (i.e. I used to get thumbnails for all of my albums, but then sometime ago albums with accents and umlauts stopped processing).
(2) I am hosting my files on an external USB drive formatted on a Mac with FAT32; are you using FAT32 on your storage, as well?
(3) MPaD and MPDroid have none of these issues, but of course they are pulling the "Folder.jpg" file from the album folder, directly.
I can send you some example FLAC files, if that helps, zipped-up to include the complete file path. On my MacBook's Drobo (HFS+), an example file path that won't build thumbnails:
/Volumes/Drobo/My DLNA Music/Hi-Res DLNA/Classical Piano/Hélène Grimaud/Water (Live)/07-Jeux d'eau, M. 30.flac
On the bdp-1, the same folder shows question marks for these chars: é è and the thumbnails fail to build for all albums from Hélène Grimaud.
But, again, I can browse by folder via MPaD and everything appears as it should.
Thanks, again, for looking into this.