Apps on Win 10 workstations suddenly saying .net v4.0.30319 needs to be installed

We have several machines running windows 10 which have suddenly started displaying errors when opening dotnet apps which previously worked. Amongst these is MSTeams but several apps on the same machines have all stopped working at the same time, so its an issue with windows and not the application itself.When you open the application it displays a message indicating

This application requires one of the following versions of the .NET framework:V4.0.30319

Windows 10 comes with .net4 installed so you obviously can't just reinstall it.In c:\windows\Microsoft.NET\Framework64\ I have a v4.0.30319 folder with what appears to be a valid .net installation. I've compared the folder with other machines and it looks ok.

If I execute the following Powershell

Get-ChildItem 'HKLM:\SOFTWARE\Microsoft\NET Framework Setup\NDP' -Recurse | Get-ItemProperty -Name version -EA 0 | Where { $_.PSChildName -Match '^(?!S)\p{L}'} | Select PSChildName, version

it returns this:

    PSChildName                      Version-----------                      -------v2.0.50727                       2.0.50727.4927v3.0                             3.0.30729.4926Windows Communication Foundation 3.0.4506.4926Windows Presentation Foundation  3.0.6920.4902v3.5                             3.5.30729.4926Client                           4.8.09037Full                             4.8.09037Client                           4.0.0.0

which, again, seems to be ok. The 4.8 version number varies on other machines

so, it looks like something has been modified which stops .net version detection from working as it should.

I've tried repairing via dism. KB5033910 is a CU for .net and win10 which I thought might reset things but apparently windows10.0-kb5033910-x64-ndp48_b1a3e722dbcd8d88b845314b8e9bf5e8fd9b6904.msu is not applicable to windows 10. Go figure.

Anyone seen this issue or know how .net is detected by apps compiled against .net?

EDIT: The MS tool CLRVER.exe returns the following output:

Microsoft (R) .NET CLR Version Tool  Version 4.8.9032.0Copyright (c) Microsoft Corporation.  All rights reserved.Versions installed on the machine:v2.0.50727v4.0.30319

and if I enable .net activation logging via

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\CLRLoadLogDir

Then when I run the dfsvc.exe tool then resulting log indicates

13904,209391.187,Parsing config file: c:\windows\Microsoft.NET\Framework64\v4.0.30319\dfsvc.exe.config13904,209391.187,Config File (Open). Result:0000000013904,209391.187,Config File (Read). Result:0000000013904,209391.187,Found config file: c:\windows\Microsoft.NET\Framework64\v4.0.30319\dfsvc.exe.config13904,209391.187,UseLegacyV2RuntimeActivationPolicy is set to 013904,209391.187,Config file includes SupportedRuntime entry.  Version: v4.0, SKU: client13904,209391.187,Found a supportedRuntime tag in the config file13904,209391.187,ERROR: None of the listed supported runtimes are installed on the machine or can be used to satisfy this request.