If when trying to login with the SkypeShield App you get the following error:
This would indicate indicates something is wrong with the External external certificate. Verify that validity of the certificate using one of the many free-to-use SSL checkers (SSL labs for example)
and test with the Lyncdiscover URL and the External Skype Pool URL
If you get a low grading (B and less in SSL labs) please fix the certificate.
If you are getting a high grade (A\A++ in SSL labs) and still getting this error please contact support.