I get this error now and then, can't access the forum at all without deleting the cookies.
HTTP 502 Proxy Error - The size of the response header is too large.
5xx server response codes "usually" indicate some sort of server or upstream problem. I in no way think it's related directly to the size of your cookie. I did read somewhere clearing the browser cache seems to fix the problem even if only temporarily. I read alot of stuff about ISA servers on the web in regards to this response code so I'm not exactly sure what the problem could be.