My PC has been on for about 1 hour (since last restart) and I have been playing Minecraft for at least half that time.
I tried restarting my PC, Router and also tried using other Minecrafts like SpoutCraft, I tried no Optifine and also have looked into the error, I believe it is an error caused by a plugin when a player does a certain thing (I forget what things can trigger it) they can get a web error between the server and the computer, in theory, my account is bugged for your server, I believe a restart can fix it, from experience, I know that not having the latest ChestShop can cause this (I think).
I can connect to almost every other server on my list.