Some more information
Thank you for the reply mistwang.
As you have mentioned in your post, I have tried to turn keepalive to 1 or 0 on both server and virtual host level and conducted some tests with people having the problem before. I have turned on the debug level logging and saw that altough keepalive was turned off, litespeed still closes the connection during ssl file uploads through php at the end of keepalive timeout seconds. It looks like keep alive is still there, or at least that is what seems to be happening in the logs.