IIS Log 500 error no url

Recently, I found my IIS log file appear those messages:
2007-03-19 16:01:11 144.135.6.240 - W3SVC1 SAPPHIRE 10.192.2.38 80 - - - 500
0 0 0 171 - - - - -

It came around 900+ times per day at one of my production server, what's it?
It looks server has token time to process and generated with 500 error, it
harms my system?

thanks for any informaiton.