XMLHttpRequest fails for files larger than a few KB

XMLHttpRequest fails for files larger than a few KB

We have a file upload script that displays progress using the
XMLHttpRequest send() to a php script for handling. It's been working for
about 2 years now without issue.
Yesterday, it stopped working for any file over about 10 KB (for all users
regardless of browser/OS). We haven't modified ANY code.
The php.ini file was modified yesterday, but to my knowledge the only
change was to add this line: "zend_extension =
/usr/local/lib/php/extensions/no-debug-non-zts-20090626/xdebug.so" We've
since removed that line, thinking that may have caused it - but the upload
problem remains.
After further testing, we've pinned it down to specifically happening
whenever the onreadystatechange is triggered with a readyState of 3
multiple times, on uploads where the file was small enough to only trigger
readyState 3 once, the file uploaded fine.
Is there something in the php.ini that could cause this behavior?