Also in firefox you can 'Ctrl + Shift + J' to see the error console.
Mainly I solve errors like these by backing tracking slowly. Including going back in time with the db - the sitetree* tables. Often things like not have the happened when an svn update has removed the page type I have configured, my page (extension of data object) has some fields which are not not configured correctly (in the way the cms is trying to display e.g. summary_fields, custom tabs entire) because when you save ss re-scaffolds the fields based on changed data)
swaiba listed it very well!. I use firebug's console panel to view the full response which lists the actual error message. The reason we don't display full error logs to the user is thats far to overwhelming (errors could be MASSIVE!) but the error should be in your console. Most of the time with error saving messages your missing database columns.
Thanks guys. It was a silly error with this 2.4.0 site, I defined the fields as HtmlText instead of HTMLText, but the TextareaField could still save into it, I didn't even check that as I'm testing the site on Windows and didn't think case sensitivity issues like that showed up. I had a 2.3.3 site I run doing the same thing last night, but that only happens randomly and can only be resolved by restarting FF, but I'll just upgrade it or something rather than bothering people here with the details.