MediaWiki: fixing “wfMkdirParents: failed to mkdir”

The server crashed again last night (yeah, when it crashes, it seems to always happen when I’m not immediately available). Weirdly, the 1and1 monitoring tool only detected 20 minutes of HTTP downtime: it thought it was back online, while in fact we had an ongoing global downtime of over 4 hours. Anyway, as usual after a hard reboot, I then browsed through the Apache logs (in /var/log/apache2) to see what the heck happened. Some insufficient memory, like last time. But I also found tons of lines looking like:

[Mon Mar 12 19:16:43 2012] [error] [client 82.165.148.117] PHP Warning:  wfMkdirParents: failed to mkdir "/xxx/images/thumb/7/7a/Hospital_Tycoon_-_bonus_code_page.jpg" mode 511 in /xxxx/includes/GlobalFunctions.php on line 2324, referer: http://www.wiki4games.com/index.php?title=Special%3ASearch&search=hopital+tycoon&go=Go

Which means that MediaWiki was failing to write to the thumb folder (the folder where the scaled down pictures are cached). Searching a bit around, I found (source 1, source 2) 2 first-line possibilities: non more space and incorrect permissions.
To check for space, use df-h. That wasn’t it, I had space remaining, even though not plenty.
We already had permissions issues a while back, because of a faulty MediaWiki upgrade script, and indeed that seemed to be the problem: although the images folder was fine, the thumb subfolder didn’t belong to the httpd user nor had 777 permissions. So I chowned it to the httpd user (syntax: chown user:group thumb -r), but another option would have been to chmod it 777 (syntax: chmod 777 thumb -r).