Hi janui,
Thanks - I wasn't aware of the other characters causing issues. FYI I don't see the problem you describe with the ':' colon character; I have a lot of classical music in my library, and the colon's used extensively in both the Artist and Album fields.
The host file-system for my Rune library is ext4, on a Synology NAS. My workflow for adding new music is a bit fiddly: I always import music to a local iTunes library (on an NTFS-formatted Windows PC) first, then edit the metadata, then copy the new folders & files onto the NAS, before finally updating Rune's MPD library. As a result the "difficult" characters are auto-replaced by iTunes before the folders & files are copied to the Rune library. I don't know if this would influence the different experiences we're seeing with the colon character.
One more related thing: the "Back" button (top-left of the library lists) seems to go to unexpected places if the library-list is empty. I've only seen this a few times so far - I'll post more information if I get a clearer picture of what's happening.
Peace and friendship,
englishtim