I can't send messages in multiplayer.

When I play in multiplayer I can send a message but only with /msg [player] or private message with a plugin or I have in red a " Cannot send chat message ". I try on my hamachi server and on a big server (eu.mineplex.com) but I still have the same probleme. It's annoying when I play with my friend.
Here is a screen of the message.
Sorry for my bad english.
I wish you can resolve my case.

• chappaz