In case it helps anyone else, I ran into this problem using the Security Tools Distribution (STD), which is based on knoppix. File locations had shifted enough that the above script would have taken some work (maybe I'll try to adapt it).

In my case, I was getting a lot of cp (copy) errors as files from:
/usr/lib/knoppix/plugins/*
had symbolic links to the cd (/KNOPPIX/???/nessus/plugins/* ).

To work around, since I don't recommend a scan with anything less than the latest plugins, I deleted /usr/lib/knoppix/plugins/* and it's subdirectory (.../plugins/.desc/*),
then re-ran nessus-update-plugins. Bingo.

Hope this helps.