EVE seems to be having yet again some problems with the chats! This time it is with joining channels (at least). When you log in you might sometimes get a random message saying that you are not allowed to join SC Comms (and proly any other custom channels you are in). This bug will also prevent you from seeing fleet chat channels when you join a fleet.
Solution is fairly simple: relog and re-join all your private channels (hoping you remember the channel names you have joined earlier)