I was seeing this kind of error in Nintex for Office 365 workflows for some time, but actually never had enough determination to check what is the root cause. However when customer started asking, I started digging.
In the first part of the series I wrote a general overview of Message and Adaptive Cards, their brief history and purpose. This part is focused only on Message Cards – the technology from which it all begun and that is still available to use although recommendation is to move to Adaptive Cards.
Some longer time ago I was asked to think about a solution, allowing my client to notify his employes, about changes and news that were being published in the application built on SharePoint 2013. However, client did not want to use alerts, or to develop a dedicated application. No – the client wanted something like a newsletter, which would allow him to sent HTML formatted messages to all users (or SP Groups of users) who were working with the application, or to anyone having account in SharePoint (or to anyone beyond the organization).