Size of a request header field exceeds server limit due to many If-None-Match values
Solution 1:
You've found the problem. Some user agents and some requests just get too big for web server defaults. It seems like a silly problem to run into but it keeps happening to me in nginx. Usually when there are silly cookie names and uploads going on... Anyway...
The solution is to just bump the request limit. You can do this globally or just for your site with the LimitRequestFieldSize directive:
LimitRequestFieldSize 32768
That's a 32KB (up from the default of 8KB) limit.