I am using SSP 1.1.1. In the Components tab of the Synergy Configuration this sets "gx" and "gx_src" to 1.1.0. I just double checked and I was mistaken about the "11". The source code actually did have "GX_EVENT_TIMER" in it, not "11".
To reproduce the problem, I have to go in and out of a window that has a timer running many, many times before the problem occurs. When I leave a window, I call gx_widget_delete(). It appears that if there happens a timer event in the event queue when gx_widget_delete() is called, the problem occurs.
When the problem occurs, I consistently find an event with a type of 11, which is GX_EVENT_TIMER, and with gx_event_target == 1 in _gx_system_event_remove().