CI CD email notifications formatting lost or empty

Two day after upgrading our gitlab ce from 15.02 to 15.05 to 15.40 to 15.4.2
Multiple upgrades i have done before without any issue.
This upgraded perfectly without any issues as well.
But then Developers mentioned that they were getting empty email or badly formatted emails.
No changes were made in the gitlab configuration before or since that could affect this.
I saw somewhere that i need to use gitlab rails console.
But I cannot find any good examples how i might debug this ?
Does anyone have an idea how i can go about debugging / solving this ?

Thank you