Hi,
It appears you have enabled Require client certificates option for your web application. The option is located in the Directory Security tab of your web application in IIS Manager. Click Edit button under Secure communications and look at the Client Certificates options.
Internet Explorer dispaly a Client Authentication dialog box for user to select a client certificate when IIS requires a client certificate for the requesting resource.
Zhao Ji Ma
Sincerely,
Microsoft Online Community Support
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