This is just a debug message. It indicates that a player was sending "mount" sync packets, but the server wasn't aware that the player is actually sitting on a mount. This can happen if there is either some desync, or if there is a high load on the server (e.g. if the server hasn't processed the "player-riding-mount-now"-packet [TCP], but already received the sync packet [UDP]).
If it only happens sporadically, you can ignore it. However, if it keeps spamming your log, there is apparently something wrong oO
