In the previous version of arma it was possible to use chat in the lobby with disableChannels[]={0,1,2,3,4,5}; in the description.EXT
This seems to have gone with the last update. (1.60)
With this issue you cant login as admin on the server which is annoying, when you go in game you still cant use it, when you go back to the lobby from game it works.
Make a simple multiplayer mission with a unit and save it, then create a description.ext file in the mission folder with disableChannels[]={0,1,2,3,4,5}; in it. Export it as a PBO.
Get a dedicated server up playing the mission, then you see you cant use the chat in the lobby. When you recreate the mission pbo without the discription.ext it works.
If needed, workaround: