Same problem here with hungarian characters not syncing on Ubuntu Linux, with btsync 1.4.91-93. 1.4.83 works fine. http://syncapp.bittorrent.com/1.4.83/
After upgrading to 1.4.91 on my x64 Ubuntu Linux box, I get the same error messages, and none of my files containing Hungarian UTF-8 characters is synced to other devices/computers. Reverting to 1.4.82 fixes the problem.