Add Managed Handler error

Hello - hope this is the right place to ask this...  running IIS7 and an ASP.NET web application under an application pool setting of "Integrated .NET CLR version 4.0.30319"

Works fine.  Except I want to add a custom handler to the project (built in Visual Studio 2017) - I tried manually updating Web.config but it didn't work (just got ignored by the application), so I then tried going through IIS itself.  So I browse tot eh application therein, select "Handler mappings" and click to "Add managed handler..."  I then get a popup error window:

There was an error while performing this operation.

Details:

Cannot execute a program. The command being executed was
"C:\Windows\Microsoft.NET\Framework64\v4.0.30319\vbc.exe" /noconfig
@"C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\[randomstring]\[randomstring]\[randomstring].cmdline".

Any help or ideas gratefully received...

[edit] more info: I have a number of sites on this machine - those running under .NET 2.0 allow me to add managed handlers; those under .NET 4.x give the above error......

[edit] it seems the folder path specified under  "..Temporary ASP.NET Files\root\" (all those random strings) is not being created - well, there is one, a new one each time I try to add a handler, but not with the path given in the error message.... 

regards

Phil