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.