When ever my friend tries to join a server in this modpack he gets a "Disconnected" message, single player working fine.
Mirror log install (2.6MB)
Install a separate instance of the modpack and try again.
You may also try at your own risk;
Disabling your firewall
Disabling your antivirus to rule out if its a network issue.
Stay home and wash them hands.
"Don’t stop when you’re tired. Stop when you’re done."
From your girl,