editing large post javascript error commentsBox is not defined

Have been running this wordpress site (http://crag.asn.au/) for more than a year. Has been working very nicely so far.

After upgrading from 3.3.1 to 3.4.2, I noticed that I could no longer edit the largest post (7,200 words) on the site. All the other posts could be edited normally.

The symptom was a blank editor page for the document. I noticed this javascript error on the page (post.php): “commentsBox is not defined”
it looks like some JavaScript files are missing, comparing it to pages where the editor works ok.

After searching through forums, I found a solution: deactivate all the plugins. The editor worked OK after that. I used that workaround to keep editing the document, up to almost 7,400 words. After that last edit adding more text, same problem again. Can no longer edit this post only, even after disabling all plugins.

Even though the editor seems blank, it does contain the document content, that can be seen by highlighting inside the editor box.
I copied the whole post (HTML markup) from the live (UNIX) site editor box into my local (WAMP) wordpress site. That post could be edited fine on my local wordpress site. My local wordpress site has fewer plugins.

I’m running out of ideas to fix this.

Any hints as to what to try to get around this issue?