It's definitely a client-side issue, however before your upgrade your Apache configuration would just accept those broken requests without error. In particular, IE6/7 are knows for broken HTTP/1.1 handling.
I wonder what changes you applied to your Apache configuration. In particular, did you use to have "BrowserMatch" directives that you didn't carry through your upgrade ?
It's definitely a client-side issue, however before your upgrade your Apache configuration would just accept those broken requests without error. In particular, IE6/7 are knows for broken HTTP/1.1 handling.
I wonder what changes you applied to your Apache configuration. In particular, did you use to have "BrowserMatch" directives that you didn't carry through your upgrade ?