Sharepoint - SharePoint Designer giving error while publishing SharePoint Workflow

I've had this too, it started yesterday I think. This Microsoft link fixed it for me.

The ​problem was caused by a .Net update. If only these things were tested before they were released!

Below is my workflow error message - all workflows had stopped working in the Sharepoint Farm.

I was told that the MS fix in the above link, running IISRESET and also a WakeUp script fixed it.

As a solution for this problem, follow the instructions in this Microsoft support article by adding the code referenced to the web.config file on your SP server. In my organization's case, this was applied only to web front end servers, but check with your IT team to verify that against the topology of your farm.

Microsoft released a .NET update which causes lots of problems with SharePoint workflows.

The article also contains a PowerShell-Script which fixes the problems caused by the patch: Add-CodeDomAuthorizedType.ps1

Simply download the script on one SharePoint-Server and execute it in SharePoint PowerShell. The changes are automatically propagated within the entire farm (if you have mulitple servers).

Important: This script causes an IIS-Reset on every server. So SharePoint will not be available for some minutes.