How to read USR1 and USR0 LED status

Some of my beagleboards (rev C3) in my network have the USR0 LEDs blink twice as fast as the speed when the beagleboard behaves well. When this happens, the beagleboard only responds PING but not “ssh”. It does not respond to the serial terminal input neither. Does anyone know what does the USR0 trying to say? The beagleboard is running the Angstrom Distribution. Thanks in advance.