It turns out that Internet Explorer caches AJAX requests (how clever). If you’re using $.ajax(), you can pass cache: false option to it, but if – like me – you’re using $.get() and $.post(), just use

$.ajaxSetup({cache: false});

somewhere at the start of your script.

Mostly for my future reference. -threads 8 should be changed to the number of CPU cores (my i7 has 4 with HT, thus I set it to 8).

ffmpeg -i input.avi -acodec libfaac -ab 48k -vcodec libx264 -vpre fast -vb 500k -threads 8 -f flv output.flv