ADUC831 and USB-EA-CONVZ

Hello,

I have been using the USB-EA-CONVZ with the ADUC831 successfully until suddenly when I try to upload code from my Keil debugger I get a Serial protocol error: NACK reply received.  I can no longer debug and set breakpoints with it but can successfully program the part using it with the the WSD tool.  Any ideas what could have gone wrong?  Thank you.