Apache: Mod_security causing 404, unless i clear the cookies every session

Question

I installed mod_security on my apache and ubunto14 server and it works, it's enabled. but my website started acting weird, throwing 404 errors then after some suggestions found on the web I cleared the cache & cookie and everything worked. until the next page reload.

what am i doing wrong? what else do i need to setup for the server to work correctly?

thanks