I was encountering this same problem before on one of my machines at home. The reason I was getting this error as well was because my testbed was a very slow machine (P133,24MB RAM). I think that PWS wasn't updating as fast as it should be.
Unfortunately there's no steadfast answer as to why this happened, as even if I restarted PWS nothing would happen. (At work, any global.asa changes I make are recognized instantly), so I too had to restart the machine.
sorry i didn't have an answer for you, but i do feel your pain.
leo