I've highlighted your problem for you.hmmm, guess I've had a lot of trouble the last 8 years... I should quit...
Prosper IS a great piece of software and even better because it's free. However, it doesn't take away from the fact that I've experienced errors with the stock JS. It only happens on Windows in IE and it doesn't seem to happen all the time. I haven't been able to track down why it happens, but like I said, I'm no JS expert.
I'm on a mac and have been for a long time. To reproduce the error I had to boot into Vista and fire up IE. The error does happen sometimes and I'm sure my install is not the only one doing it. Yes, it could be something simple like doctype or content-type causing it. Fuck, it could even be the fact that I'm running Windows in VMWare, who knows. Either way I wasn't able to fix it or figure out why it happened.
If you have a little budget check out DT's one they're releasing soon, Directtrack.com - Tracking Software.