500 Server error: Premature end of script headers:

I have a website in php that works with a solr indexation server, based on CodeIgniter.

We got a lot of new content, so we flushed the database, and had to reindex the content (about 168,000 elements). I created a script to index the contents by slices of 500 – when the script finishes, we launch the next indexing.

It works perfectly on my local test environment, but on production I get this 500 error:

[Thu Dec 02 ...] [error] [client IP] Premature end of script headers: index.php

There is absolutely nothing in my php.log, just the apache error_log that returns it. I've seen it happen on other pages of the website once or twice, but it was during this indexing.

Any ideas?