Thanks for posting, can confirm that this helps and is certainly the quicker workaround than rebooting. Will try to see if I can move the issue I created with the .NET people to a more appropriate repository. Even so, the underlying cause is the OS upgrade, since .NET executables didn't change at the time, even though it only seems to appear with .NET. Will be interesting to see if any other access patterns appear through a different set of binaries

It asked me to "Allow", "Always Allow" - it seemed to work. Can I maybe revoke that somewhere and then save myself the reboot cycle?

"A problem with the keychain itself" - are there any procedures to mitigate that?

The code is running in the context of a process running under the "dotnet" executable. See also my answer.