[TYPO3-english] Error Not Acceptable on Click enlarged images

[Alwin Kruijt]

Dear all,
I have a peculiar problem on one of my installs. Suddenly all images with the click enlarged option in the editor selected are displayed as 
Not acceptable.
An appropriate representation of the requested resource /index.php could not be found on this server.

Example: any image on http://seafoodplus.org/Popular_articles.327.0.html.

I mirrored the website to a serparate domain on the same webserver (http://seafoodplus.nl) to fiddle on.
Upgrading from 3.7.0 to 3.8.0 to 4.2.10 doesn't help except for screwing everything that needs upgrading like news and filedownload plugins and stuff. I upgraded all major extentions .
Then I did a clean 4.2.10 install using CPanels Fantastico DeLuxe into a subfolder http://seafoodplus.nl/new, copied all images, templates and stuff, connected the database (so 1 database with two T3 installs and the error in the /new install is gone. However so is 80% functionality because all plugins, templates etc. don't function or are no longer available in 4.2.10.
In the last month of testing I tried checking the differences in the two installs, but I can't figure out what part is causing the error. 
I guess it's not in the database, they're the same (two installs pointing at the same database).
The files in /fileadmin and /uploads are the same, except that the not working version is an upgrade against a fresh install of the Fantastico version in /new.
It's the exact same sever, just a subfolder, to the server is not the problem either.
I disabled step by step all extensions, with no effect on the error.

Creating a clean install is something I don't want to do. I spend six months five years ago to create it as it is.
I'm only supporting the website for small errors, so spending months to re-learn T3 since version 3.7.0 is a no-go for me i'm afraid. 
I have only 1-2 hours a day to fiddle.

Anyone got a clue what is causing the error or were I can start the search?
Paid help on a small budget is an option for me at this point.

Alwin