Boost crawler did not get a 200 response; -24 returned instead.

I tried installing Boost this afternoon on one of the sites I'm building. The install seemed to go smoothly, and I'm able to see pages being created in the cache directory on the server. However, I notice on the site status report an error for Boost with the following text:

"Boost crawler did not get a 200 response; -24 returned instead."

This is followed by advice to clear cache and run cron, and file a report if the problem persists. I cleared the cache (both Boost and core cache), and ran cron. The problem persists, so here I am.

I can provide a list of additional modules in place if necessary. Drupal itself is on 6.19.

Additional data points:
* When I browse to a cached page as an anonymous user and view source, I do not see the last line of the source code that indicates the page has been served from cache. When I add the page status block to my footer region and browse as an authenticated user, however, I see a sentence that gives cache lifespan. When I inspect the cached page (in the cache directory) in a text editor, however, I see that last line. This indicates to me that the cached page is not, in fact, being served.

* I had previously had to add a line to the settings.php file to get around an "HTTP Request Status" error. Now, however, I see that error has re-appeared on the status report as well; so I wonder if this is related. The line I added to settings.php reads:

$conf['drupal_http_request_fails'] = FALSE;

This came from reading this: http://drupal.org/node/245990#comment-1312270

Any help or recommendations on how to proceed would be welcome - I searched the issue queue for Boost and found several tickets, but none seem to match the "error -24" that I'm getting.