All the other channels work fine, including say and yell. But nothing goes on General.
I quit and go back to general, just to be sure, but that does nothing.
No lua error.

Update: LocalDefense also unavailable.
And something weird in the base UI:

I just tested on all clients and all of the channels are working properly.

Are you sure that you're in the channels when you send the message? If General is showing up with a Join button, I think that means you're not actually joined to it. The weird thing is that General doesn't show up in both spots on my clients, maybe you have encountered a bug in the client itself:

More tests:

• the addon can't send messages in channels where the join button is still present (general and local defense), despite the fact that i'm allready joined in.
• i can send messages myself in said channels

It feels like a base UI bug (i tried with no addons, the join button bug is still there), but strangely, it's working - except for the addon.

closing as there is no reproducible bug here