What is happening?
After applying January Patches to Windows Server, when you try to go to the Recast Management Server interface you get a page saying there's a 500.31 Error. When you attempt to use the Right Click Tools, you will get an error saying that the tools were unable to connect to the Management Server.
Why is this happening?
Beginning with the January 2021 Server Patches, Microsoft has started updating the .NET Core Framework components as part of the releases that are available through WSUS (and therefore ConfigMgr). Unfortunately they were not able to include the Hosting Bundle which is one of the components that is used by the Recast Management Server. Mismatched .NET Core components will cause an error, and cause web services to stop working.
Microsoft information on what is happening is located here: Dot Net Core Updates Coming To Windows Update (Microsoft.com) There is a note on the page: "Note: updates for the Hosting Bundle won’t be available on MU initially while we work through a couple of issues. These updates will need to be downloaded and deployed from our download website. We will update this post once the issue is corrected and we are ready to start delivering updates for the hosting bundle via MU."
How do I make sure I'm having the same issue?
To verify, check in Programs and Features on the computer that is running the Recast Management Server. You will see various versions of the .NET core components as illustrated below.
How do I fix the error?
To fix the error you will need to download and install the .NET Core Hosting Bundle with the same version number as the Shared Framework (3.1.11 in the above screenshot).
The Microsoft page with the updated version of .NET is located here: Microsoft.NET Updates
Be sure to download the "Hosting Bundle" identified below.
NOTE: This may happen again!
Please note that if Microsoft is not able to fix the issues that they are currently experiencing with pushing the update of the .NET Core Hosting Bundle, this same error may occur in the future. You may need to download the newest .NET Core Hosting Bundle if it is not updated with the other components in future releases.