Since the update to 1.6.1 I have been unable to type the following letters in any in-game chat:

"^"
"å"
"ä"
"ö"
"§"

These are the letters I have noticed non-responsive so far, though I think there are more.

Note that for all of these except for "§" which won't appear at all you are still able to paste them into the chat by typing them outside of the game.

Note that the issue clearly does not lie within my keyboard, seeing as I just typed the non-responsive letters in this report.

The reason I am certain this is a larger issue is because a friend of mine has the exact same issue whilst playing on his Mac, though his versions I have no way of checking to see if they match.