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 a post 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.
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 <IfModule mod_security.c> SecFilterEngine off </IfModule>
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.