Try <sessionState mode="InProc" cookieless="false" timeout="20" />
for reference remove AspxAutoDetectCookieSupport
or use this in a web configuration,
<authentication mode="Forms"> <forms cookieless="UseCookies"/> </authentication>
Looking through this problem, I found in one article
You can change cookieless="AutoDetect" to cookieless="UseCookies" . This will get rid of it, but users without cookies will not be able to transfer session objects around. Depending on how you use it, it may or may not matter.
You can also write a routine that sniffs search bots and rewrites Url, or write .browser files for search engines about.
For reference: Remove cookie support
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