There is currently a .Net update that has caused disruption on SharePoint 2007,2010,2013 and 2016 platforms.
After applying .NET Security Only patch to resolve CVE-2018-8421 (Remote Code Execution Vulnerability) , all SharePoint "out of the box" & Nintex Workflows fail to execute and the log will show an error like the below:
System.CodeDom.CodeBinaryOperatorExpression is not marked as authorized in the application configuration file
Publishing a Nintex workflow
Microsoft link to issue & resolution.
Microsoft related update information:
Microsoft .Net 3.5 Information.