Starting last Thursday all my users with Office 2016 pro starting getting a security warning when opening attachments. It won't take their user name or password but they cancel it and all works fine until they close outlook and start it again. I installed on three new laptops all have the same problem. Then a user I installed 6 month ago started having the same issue. We do have our own exchange server 2013 and Certificate is issue through Network Solutions and has not expired. Anyone else having this issue or heard of it?
Attached is screen shot from user.