Hmm.
Could you check in your NonCGI directory to make sure that the Forum95 directory does indeed exist?
The error is "impossible" - it's triggered when the attempt to create the Forum95 directory fails.
You can see that the error is "File exists", which basically means that the directory couldn't be created because it's already there.
However, the code to create the directory is only called if the previous check to see if the directory exists failed.
The directory can't both exist and not exist.
If the Forum95 directory exists, please try the upgrade process again (click the button in the control panel).