I have 2 Software applications:

• An interface software that normally should run as a Windows service
• and an emulator whose goal is to test this Windows service

Both are

• written in ASP.NET Core 3.1,
• both use the Kestrel web server,
• both can act as a client and as a server to each other,
• both automatically choose an appropriate certificate from certificate storage (Current User or Local Machine).

To test things I generated a self signed certificate for localhost and installed it in Local MachineRoot CA.

Everything is fine when interface and emulator are running as normal applications. They can perform a bidirectional communication.

When I start the interface as Windows service, the emulator can connect to it, but when the interface software uses its client (a descendant of System.ServiceModel.ClientBase<T>), it cannot connect to the emulator. The error is:

Could not establish trust relationship for the SSL/TLS secure channel with authority ‘localhost:44357’

Please feel free to ask for additional information, because I don’t know which information could be relevant.