It appears that my webhost has done some server upgrades and must have tightened up security a little. This resulted in one of the pages on my site being prevented from being saved - every time I would hit "Save" or "Save & Publish", it would result in "Error Saving Page".
The error console revealed that it was a "403 Forbidden" error on /admin/EditForm. It reminded me of an earlier issue I was having with the same page for which I made [url=http://www.silverstripe.org/general-questions/show/18852]a post[/url] in the forums. I suspected that there was some filtering going on somewhere (wasn't sure where at that stage) as it was picking up the word "system" in alt/title tags and causing the same "Error Saving Page" issue.
I stumbled across [url=http://www.silverstripe.org/general-questions/show/5614]this post[/url] which made me aware of a [url=http://doc.silverstripe.org/old/suggested-web-hosts]web host issues wiki page[/url]
To cut a long story short, by including the following lines in the .htaccess file, I was able to eliminate the problem:
# Disable mod_security to prevent restrictive calls between the server and client
It turns out that the content on that page must have formed a signature that triggered mod_security, preventing it from being saved. Other pages were fine.
Anyway, I hope that info helps someone who might be pulling their hair out with similar issues.