When you try to ride the horse (tamed or untamed doesnt matter)you get disconnected directly with the following error: Internal exception: java.lang.NullPointerException
The server says: [SEVERE] Reached end of stream followed by the player disconnecting. After the error the player cannot relog again simply getting the same error the second he/she logins. The horse he/she appears on gets duplicated aswell. Needed to delete the <playername>.dat from the world folder to be able to login again. Also the second he/she mounts onto the horse the server the cpu usage increases from almost nothing to 100%