you can try to remove the cert from the default website. This is done by going back into Directory Security and clicking on server certificate and going through the wizard to remove. this should then not require the default website to have a certificate. You can always re-apply the cert by doing the same steps but choosing to assign an existing certificate. You do want a certificate on the site though so this certainly is not a fix.
I also wouldn't do this unless it was affecting every user. If the issue is with just one user then I would install the certificate on the users phone. this link will walk you through that
http://technet.microsoft.com/en-us/library/aa997575(EXCHG.80).aspx