So, an age old problem mentioned dozens of time in these forums starting from older versions (i think even prior 2.6).
A user types a message, hits Enter. The message is sent to the recepient, but the user still sees it in the input field and chat window doesn't show it as sent. And the user usually can't see incoming messages either. Often a user can close a chat window, open it again and then see message in the quick history. Usually a user needs to restart Spark to fix this.
So, as i have rolled out Spark to 200 users, i know get reports about such problem. So far about 10. Will try to investigate this and update this thread. One report was interesting. A user reported that she had that problem yesterday and tried to restart Spark couple of time with no success. But today everything is fine. So i suspect the history xmls and in particular the "current" one. This file stores recent messages and then they are moved to the whole history file. Maybe something gets bogged down in this file and after some time it fixes itself after it moves history to another file.