I was having this problem with apache 2.2.9 and upgrading to apache 2.2.22 solved the problem for me.
I would get random 400 response codes on some of the page requests. Sometimes images, sometimes AJAX requests. Occasionally it would be the page itself that would return a 400 along with this message.
Bad Request
Your browser sent a request that this server could not understand.
Request header field is missing ':' separator.
live
I found the following bug report for that seemed related. The bug was fixed in apache 2.2.15 : https://bugzilla.redhat.com/show_bug.cgi?id=652335
I think this bug is the cause for lots of random errors when using https:
Your browser sent a request that this server could not understand
Request header field is missing ':' separator.
So thought I'd try updating to the latest apache - and that seemed to work.