This has caused an even bigger error than I originally realised, literally.
I received an automated email to tell me I was running out of space on my webspace. On checking, there's an error log in the root, which is over 500 MB in size! It is full of nothing but 1000s of lines with the following:[30-Jan-2012 16:56:55] PHP Warning: readdir() expects parameter 1 to be resource, boolean given in /home/domain/public_html/libraries/joomla/cache/storage/file.php on line 501
The time co-incides exactly with when what I wrote in my original post above happened. So, it seems that enabling (or later disabling) the caches caused this problem. Whether it was Gantry's or Joomla's cache, I have no idea.
What do I do about it? How do I get rid of this huge error log? Which cache do I therefore set - or not - when I go live in a day or two?
This is URGENT, please help.