We have had several instances of a V2 Azure Function, which contains a few timer and blob storage triggers, running for the last 2-3 years. They're deployed in different resource groups in different Azure regions around the world. Looking back through their logs, they all stopped working within the same two hours on the evening of 21st May 2021. The solutions continue to work locally with no problems when debugged. They all began throwing the same exception each time the triggers were invoked:
Exception thrown: 'Microsoft.Extensions.Options.OptionsValidationException' in System.Private.CoreLib.dll
Exception thrown: 'System.Reflection.TargetInvocationException' in System.Private.CoreLib.dll
It seems to appear during the Startup.cs, throwing a Dependency Injection options validation issue: "Exception has been thrown by the target of an invocation. DataAnnotation validation failed for members: 'DeviceConnectionString' with the error: 'The DeviceConnectionString field is required.'." - the question is why this has suddenly started happening with no code changes made to any of the deployed Azure Functions, and why they continue to work locally with no issue.
They're .NET Core 2.1 and they're marked with FUNCTIONS_EXTENSION_VERSION of ~2.0 in their Azure settings.
The solution and Azure Functions work perfectly when running locally on our dev boxes. This leads us to believe that something changed in the underlying compute environment for V2 Azure Functions on the evening of 21st May. Could anyone shed some light on this? By the looks of the error and our investigation into it, it looks like a NuGet package version was changed or updated which the Azure Functions SDK uses in the Azure Function environment - could this be the case? Is anyone aware of an upgrade of the V2 Azure Function environment in May 2021 which could help us figure this one out?
We have tried updating the NuGet package references of the Azure Function project, but have then run into a series of issues owing to a common dependency on the NuGet package Microsoft.Extensions.Configuration.Abstractions from both (1) Microsoft.Azure.WebJobs.Extensions and (2) Microsoft.Extensions.Configuration.AzureKeyVault , we get exceptions for missing Microsoft.Extensions.Configuration.Abstractions.DLL. We're tried the solution outlined here but to no avail - even though all versions of the NuGet package appear to be 3.1.0, we still get an exception that Azure Functions cannot find the DLL: 
Any help is greatly appreciated - first in helping us identify and fix the first issue to get the Azure Functions back up and running in the short term. And what versions of NuGet packages we should be using to solve the second issue and ultimately update the Azure Function to the highest version we can which still works would be much appreciated.
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