[CaptainKen]

Try restarting XBMC, maybe the Remote Communication Server has died in XBMC as this is required for JSON.

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Here's my command prompt runs:
1st, 2nd, 3rd and 4th runs

Here's the XBMC log after the 4th run:
XBMC Log

This is the 5th run:
TC.log

EDIT:
The fanart for "Big Buck Bunny" is one of the problem artwork that isn't refreshing.

Thanks for the log... I can see there might be a logic bug (or platform behaviour difference) when forcing a re-cache of items, which is resulting in deleted items not being re-cached correctly, so now it's only downloading 10 items which presumably were populated by you browsing the GUI, and not the script. If you add "@Download.payload=yes" to the commands used for your 5th run, it should re-cache all 82 items correctly (please confirm with another log - many thanks).

Thanks again for your dedication in assisting me!
tc.log

BTW, I mistakenly said one of the issues was with big buck bunny artwork, but it was actually the "blender open movies" fanart. Sorry.

EDIT:
I should have mentioned that this last attempt still did not resolve the issue.