On mission start in a multiplayer game the chat works fine.
But after respawning a few times (seems random) the chat becomes broken and doesn't work at all.
Any information that should show up in the bottom left corner (death messages, player chat, AI call outs, etc...) becomes invisible after death & respawn.
I think that I may have sorted out my problem...
A feature was introduced called 'Stream Friendly UI'
And it was set to 'enabled' as default, disabling this seems to have rectified my problem and the chat is working as it should again.
Just in case anyone else has this problem here's a link to the feature and how to switch it off...
http://thezombieinfection.com/wiki/index.php/Stream_Friendly_UI