Windows Update and assembly issues - impacting Sitecore scheduled tasks

My company recently updated the Windows servers with the latest Windows updates to patch some vulnerabilities with IIS. It wasn't supposed to cause any problems with the sites running on the servers, however one of my sites was throwing the following exception:

Exception: System.ServiceModel.FaultException
Message: Server was unable to process request. ---> Loading this assembly would produce a different grant set from other instances. (Exception from HRESULT: 0x80131401)
Source: mscorlib

From reading on the internet, I found a slightly older post talking about the fix (, so I decided to try his approach.

Stopped IIS - both the application pool and the site
Delete Temporary files: c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\ **
Restarted both the website and the application pool 

**NOTE: Temp files may vary within the Microsoft.NET folder depending on setup.

 Unfortunately, this did not resolve the issue, so I moved on to solution #2. Adding a trust level of full to the web.config. This was talked about

No luck here either!

It turns out the error was not from my Sitecore servers at all, but from the web service that we were calling. The message just happened to bubble up to my server so it appeared like my server was the issue. Using SOAPUI, I was able to call the web service directly and see that it was the producer of the error message.

The final cause - turned out to be removing the temporary files on the web service server. Originally thought it was something impacting classic mode of the application pool, but switching it simply removed the temporary files.