Searching on 404 page returns back to 404 page back

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IIS7

ASP.NET

IIS

web.config

We have a 404 page. When we are in 404 page, we have search box. if we try to search from search box, it again returns to 404 page. How could we handle this?

PS: My search button is inside user control and user control is inside masterpage.

When we go to the site below; http://www.x.com/dk

it goes to 404.

then if we make search from th,s 404 page it directs us to the link below; http://www.x.com/dk?404%3bhttp%3a%2f%2fwww.x.com%3a80%2fdk

my web config sets

<customerrors mode="Off" defaultredirect="Error.aspx">
   <error statuscode="404" redirect="PageNotFound.aspx" />
</customerrors>
 <httperrors>
 <remove statuscode="404" substatuscode="-1" />
 <error statuscode="404" path="/PageNotFound.aspx" prefixlanguagefilepath="" responsemode="ExecuteURL" />
 </httperrors>