If you are passing a large amount of XML data into the cookie it might be the problem. According to Cookie Central's FAQ
there is a 4k size limit on HTTP Headers, which Cookies are a part of. Granted, 4k is massive for a header, but, passing XML data via a cookie could hit that pretty easily I suppose.
As I stated as a reply to your other post today there might be other ways of accomplishing that you are after. Having XML stored locally could get really out of hand from a security stand point.
How about you give us a full overview of what you are trying to do, and we will offer you some advice on the best way to do it overall.
Are you posting in the right place? Check out Where do I post X? to know for sure.
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Want more info? How to link or
or How to display code and escape characters
are good places to start.