For a fresh log into the game until one uses /t [PlayerName] [message] for at least one time the Tell mode of chat box will refuse to cooperate. Sending any message by typing PlayerName, then space and then some message always fails with “No such player online…” despite /who claiming otherwise.
After I used /t FirstPlayer [message] I could use Tell mode of chat box to whisper to OtherPlayer as well, suggesting this might be some initialization stuff not working properly.
Bonus: A suggestion from Chance. He says it would be very good for clicking a player name in message log would quickly switch to whispering them.