[TYPO3-english] Upgrade to 4.2.6... BE still old version??

[Andrew Plank]

Hi knowledgeable Typo3 upgraders!

I've performed a whole bunch of Typo3 upgrades from all kinds of 
versions to all kinds of newer versions, but the most recent upgrade has 
not gone well.

Everything in the front end looks pixel-for-pixel identical, no errors, 
HTML output is (almost) absolutely identical...

When I log in to the backend, it looks like 4.2... Until I edit a 
content element. The it looks like the old version, (I think it was 
4.1.5 before)

Here's how it looks now:
http://www.imagebucket.net/bucket/is.php?i=4921&img=upgrade_broken..jpg

And here's how it should look (from a successful upgrade on another site):
http://www.imagebucket.net/bucket/is.php?i=4920&img=upgrade_nicenes.jpg

I'm familiar with Typo3, having written umpteen extensions, and know my 
way satisfactorily around TypoScript, so thought a quick sortie through 
the TS Object Browser would zero me in on the problem... But it appears 
that the inner workings of the backend are far too mysterious for me. I 
haven't a clue where to even start looking. (Well... not strictly 
true... I'm certain the TCA will be involved, though quite how or where 
its role is responsible is the thing I can't discern.)

Another thing I should mention is that the site in question was 
installed and configured by someone else who aparrently has a penchant 
for installing every attainable extension. Is it possible that one of 
these myriad extensions could be the cause of my BE grief? If so, how 
might I go about discerning which particular one of them is the cause?

Any tips or pointers would be most welcome!