I had similar problem but only in this case:
First run of isogen module in Caesar was successful and I can open iso in autocad.
If I want to make some changes for iso output during the next session of generating isometrics I got a notification error in PCF file.
I discovered that an error occurs only if the previous stress drawing in AutoCAD is opened. If I close the drawing that was previously generated at the next session in isogen there is no error.
I do not know whether it is a common problem or is it only happens to me.