If you have the Hide Players: All option turned on and you die after a rez you come back in as a hidden player and are unable to summon a mount again. The only way to clear this is by zoning. Toggling it on/off while in zone still leaves you as a hidden player.
Steps to reproduce: Have "Hide Players: All" toggled on Die but don't release Take a rez You are now a hidden player and unable to mount until you zone.