Thread:Project:Support desk/Update to 1.21 broke some pages

I just updated a wiki from 1.20.3 to 1.21.1 >> http://www.subsurfwiki.org/. Now I can read and edit pages, and image upload works. But some pages throw an HTTP 500 error, e.g. http://www.subsurfwiki.org/wiki/Wavenumber

The page exists and I can see the edit page: http://www.subsurfwiki.org/mediawiki/index.php?title=Wavenumber&action=edit (you have to be logged in to edit, but you can see the source at least). If I remove certain parts of the content, I can save the page and everything is OK again... but I can't tell what the problem is and have no idea how many pages are broken like this.

My upgrade method: unpack the new stack in parallel; copy over LocalSettings, extensions, etc.; go to read-only; switch the directories; run update.php; copy over images directory; go back to read-write. Everything in my installation is owned by the webserver user. All this has worked for me through previous upgrades since 1.18. This wiki is running on an up-to-date Amazon EC2 instance running Ubuntu, with a Bitnami LAMP/MediaWiki stack.

Any help appreciated. I am at a bit of a loss.

Cheers, Matt