Insert template shows error and Subject is not copied "an error has occurred. Please return to home page and try again"

We have CRM 2016 On Premise, we recently installed - 

https://support.microsoft.com/en-gb/help/3205084/december-2016-update-for-dynamics-365-online-and-on-premises

After the upgrade, when we try to insert email template in email it shows an error, and subject is not copied. Only body is copied.

Any help would be appreciated.