If you don't care about the events and are in a rush: I would just disable and enable the driver and if that does not work I would also clear DirXML-DriverStorage attribute on the driver object

You are running RRSD in multithreaded mode.

Each event that is received gets its own thread to the limit (500 is the max).  To track the events, it is accepted, given a thread, recorded in DirXMl-DriverStorage.

The RRSD in memory holds all these events and should be processing them.  As one comes free, and is completed, it is removed from DirXML-DriverStorage.

In your case, you hit event 501, now what should happen is a retry while it waits for some oft he 500 in flow finish up and free up a thread.

Restarting as suggested, and deleteing DirXML-DriverStorage loses the events in memory, and loses the backup about them.

But if you have more than 500 events ocming you may hit this again.  IN some ways the answer is wait.