Go into multiplayer settings and check if you have the function Chat:Command Only or Chat:Hidden on. Just change that to Chat:Shown if that is the problem.
Yes, the only cause I am aware of for that error is if you disabled chat in your client's multiplayer settings. If you follow the above advice it will resolve your issue, just change it back to Chat:Shown and you will be fine.
Chat was on and I got this during a lag spike though could that have been the issue? I wouldn't have posted this if my chat was off
Oh yeah, I found a dent in my router yesterday but it wasn't related to it. Guess I'll just have to scoot over to challenger to get a new router...
I know what causes this. If you have such thing in your "Modified Client" called Lagger or something along the lines of that, turn it off, or if you are legitimately lagging try to speed up your internet. Disconnect any devices which aren't in use of the internet and close other programs using internet. If you lag and then die and try to send a chat message you will get the same Popup. To test this out yourself, stand in the middle of a game and open chat then when you get killed try to type something. You will get that Popup/message. Hope this helped.