500 internal error on upload?

Hopefully the info below will help admins troubleshoot this...

I've had these mysterious 500 Internal Server errors happen twice now, both times for me I had the following situation:

I created a WP site that allows users to create posts via Gravity Forms on a development server with MediaTemple's Grid Service. When the site was finished, tested and working perfectly, I transferred it to the client's server, again a Media Temple Grid Service account. After doing so, the form would no longer submit, I just get the ambiguous 500 Internal Server error.

TROUBLESHOOTING STEP 1: INCREASE PHP MEMORY:
RESULT: still getting the 500 error

I have the PHP limits increased in WP to 64MB and MediaTemple states that all Grid Service accounts are set to 100MB on their end as well. MediaTemple does not have good things to say about Gravity Forms, by the way...apparently they get this a lot.
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TROUBLESHOOTING STEP 2: MONKEY WITH MY FIELDS:
RESULT: no more error, but the form can't do what I need it to do in this scenario

I have three Post Fields setup: Body, Custom Field, and Title. If I remove the Title post field, the form will submit, but that's not particularly useful to me to create a post with no title. I only mention this as perhaps some Rocket Genius developer will glean something from it.

TROUBLESHOOTING STEP 3: LOOK AT THE ERROR LOGS
RESULT: everything's working! But I had to edit WP core files and really don't want to ever do that, ever + it seems to prevent the notification emails from being sent, at least to the admin one (didn't test user notifications as they're not part of this particular form)

I've pasted the error logs below, and some testing on my part revealed that the post won't submit either on the front end of the site, or in the preview.

ERROR LOG:

[Wed Apr 04 06:03:54 2012] [error] [client 72.250.555.555] ALERT - canary mismatch on efree() - heap overflow detected (attacker '72.250.555.555', file '/nfs/c10/h01/mnt/xxx/domains/mydomain.com/html/wp-includes/class-phpmailer.php', line 677), referer: http://mydomain.com/upload/
[Wed Apr 04 06:03:54 2012] [error] [client 72.250.555.555] Premature end of script headers: php-legacy, referer: http://mydomain.com/upload/

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So I went into /wp-includes/class-phpmailer.php and simply commented out line 677, which read:

$rt = @mail($to, $this->EncodeHeader($this->SecureHeader($this->Subject)), $body, $header);

So in the end, I still don't get the complete functionality, in that notifications aren't sent by the form.

With those error logs and the info above, hopefully an admin or developer here will respond with what's happening.