Overview
Recently, a customer came to us with an urgent issue: all their SharePoint workflows had stopped working overnight. Aside from a few recent server updates, the customer had not made any changes to their SharePoint environment. One of our consultants performed initial troubleshooting using the error message and identified the root cause of the issue. Following a recent .NET Framework security update, several .NET types used by SharePoint workflows that were not previously required are now required..Net Framework
This article from the .NET Blog addresses more information on the security updates for CVE-2018-8421 – Windows Remote Code Execution Vulnerability.
Microsoft Support
This article from Microsoft Support provides a more thorough technical explanation of why SharePoint out-of-the-box workflows stopped working.