If you move all open events into history so that there are 0 remaining, then ZApplet's tray icon will change to a red & white warning sign, the pop-up will list part of the error message, and the log file will list the following:
Traceback (most recent call last):
File "zapplet\event.pyc", line 56, in doit
File "zapplet\event.pyc", line 110, in load_events_from_server
IndexError: list index out of range
(Looks like a simple fix.)
The program will continue operating normally, except the tray icon will not change back to a colored circle when new open events are available.
Strike that last bit--the tray icon does return to reporting the current status after new events arrive.