Answer
Don't create a custom event directly against HISTORY_LOG_TAB — use the predefined HISTORY_LOG_MODIFIED event instead. HISTORY_LOG_TAB is written to by the framework's own internal history-logging trigger mechanism, and adding your own custom event trigger directly on that same table conflicts with it in a way that breaks normal history logging; HISTORY_LOG_MODIFIED exists specifically to let you react to history-log activity without touching the table's own triggers directly.