Exception Name:FailedToImpersonateUsersException, What:There was no certificate provided by the client while trying to impersonate users. Either provide a trusted client certificate with impersonation rights or login with requested credentials.
Getting this on Coveo for Sitecore diagnostic page. As far as I know we have not changed any configs.
Did you search for this error on this site before asking a new question? There are numerous other questions with the same error than yours. Did you try the solutions proposed in the other questions?
According to the release notes of Coveo for Sitecore, this problem was fixed in the June 2014 release of the product. If you run an older version, you should upgrade to fix the issue.