Outlook 2016 security warning asking for credentials

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.
Chris SlackIT ConsultantAsked:
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Valentina PerezExchange ServersCommented:
Hi Chris,

You are having this issue only with Outlook 2016?

Could you please attached the screenshot?

Regards
Valentina
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Chris SlackIT ConsultantAuthor Commented:
I don't know why it didn't upload earlier?
image001.png
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Valentina PerezExchange ServersCommented:
Hi Chris,

You have only this credential propt when you tried open attachment?

And if they introduce the user and password, selecting "remember my credentials", they continue to have the same message?

Regards
Valentina
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Chris SlackIT ConsultantAuthor Commented:
The first time we open Outlook, we tried to open any attachment .xlsx or pdf this message pops up. If we enter our username and password it comes back right away. If we cancel it everything works fine until we close Outlook. Then it starts all over again if we open Outlook.
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Valentina PerezExchange ServersCommented:
HI Chris,

I think it is regarding some settings in Exchange server side.

Could you check IIS to verify that the binding are correct with the correct certificate?

Regards
Valentina
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Hemil AquinoNetwork EngineerCommented:
I had this problem before and the way I dealt it before was going to the user profile and delete outlook credential and log out.

Also you might check:

1- Make sure when you create the user has the right UPN in his email and also active directory.
2- Make sure haven't change anything in your exchange virtual directory.
3- Like I've mentioned before. Delete the credentials for that user log out and try out again.

That would work 90%
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Chris SlackIT ConsultantAuthor Commented:
I just tried to login and check the bindings. I couldn't get in by remote desktop. So I've issue a command to restart the start. Let see if the error is still there tomorrow morning when the user start to work again.
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Valentina PerezExchange ServersCommented:
Hi Chris,

Ok perfect. Maybe the issue is solved tomorrow.

Regards
Valentina
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Valentina PerezExchange ServersCommented:
No more replies from the author,issue seems to be solved
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