Certain image upload trigger 500 error; no log

Question

I'm having a very strange problem were one specific image file is triggering a 500 error when uploaded via a form.

However when I open the image in photoshop and save it again; even though the new file size is larger it uploads with no error.

The PHP and apache error logs show nothing regarding the 500 error; even with apache error logging set to debug

The file is about 2MB and the PHP upload limit is set to 5MB (post limit of 8MB)

Answer 1

I realized that because the request was over SSL; the errors were being logged in the ssl_error_log instead.

It turns out the upload was being blocked by a mod_security rule.