It's couldron server with newest build.. I reported this issue there, but my thread got closed. So now I'm here.

There is console output: http://pastebin.com/Tx9sty2V

There is what's going on in server's console while this player is trying to join:

[23:38:23 INFO]: UUID of player Cass is 54854d47-def8-34f2-b35d-cdc58a56d147

[23:38:53 INFO]: Cass lost connection: Timed out

There is something wrong with his client side that is causing to much lag between connecting to the server and getting into the world.

You may need to have him test local servers, like single player see if that lets him connect, if not he may need to trim down the number of mods he has installed.

Other (non modded) servers and single player works fine for him. But I and my other players have no problem to join the server with that amount of mods..

If it's specific to your server, then it means that something on your server end is not sending the person the login_success packet which is not letting it get past the login stage.

You would have to sit down and debug it by trying to see if any of your mods are delaying that in any way.

Have you tried with a pure forge server,no cauldron involved.

And again try removing the mods until your issue goes away.

There is just to much there that we do not control to be able to support it.