avisynth(+) + avslib - cropping each frame with different values
FirstFrame      = 1
......
......    
Raw = ImageReader("GOPR%04d.JPG", FirstFrame, LastFrame, FrameRate)


Probably the file GOPR0001.JPG doesn't exist in your current directory.

From AviSynth manual:


  The first file in the sequence, i.e., corresponding to 'start', MUST exist in order for clip parameters to be computed. Any missing files in the sequence are replaced with a blank frame. 


For testing it directly in your script, temporary put Raw  __END__ at the end of the command, i. e.

Raw = ImageReader("GOPR%04d.JPG", FirstFrame, LastFrame, FrameRate)  Raw  __END__


and play back your script (.avs file). If you get the same error, non-existence of the file GOPR0001.JPG is your problem.