When a player dies then logs out after the gui panel pops up then respawns after relog a 'pc properties' skin gets placed in their inventory.
This is happening with polymorphed characters relogging too.
Its because your server has ILR or ELC verifications on and each of them is for some legacy reason destroying the player items in creature slots. Game is then recreating a new skin, but if player is dead or polymorphed game fails to equip it.
This is solved in community patch 1.71 with a 3second recurse for delayed equipping, but
there is also another problem is when player has full inventory in which case the new skin drops on ground.
This is solved in 1.72 beta.
Either way, without 1.71, the horse system tends to mess up mounted status after relog (idk if server restart must happen or not). Losing the skin = losing the data about being mounted and thus relogged player will end up being on horse and unable to dismount. At least in my tests this happened.
There are more horse-related issues (mounting horse in unaccessable area, skill decrease stacking...), but I think those were fixed in 1.72beta not 1.71 yet.
Anyway, the best solution is not to use ELC/ILR but well... its quite important for multiplayer as without it you need to protect your server against cheaters on your own. Custom ILR is on the other hand not so big deal, there are good choices you can find on vault.