Every so often (almost every restart) Sync complains that it failed because the password was bad. On clicking on the yellow status bar at the bottom, I enter the same password and it all works... until it seems to lose it again.
I do use private browsing on a daily basis. Firefox is not set to clear history on exit, although third-party cookies are set to clear on every exit. Sync settings are preserved, just that password appears to be messed up.