Debug public key authentication for OpenSSH server on Windows

• Stop the sshd service
• Open a PowerShell console with administrator privileges
• Type 'sshd -d'
• Try login from my client machine

The public key needs to be C:\ProgramData\ssh\administrators_authorized_keys

This file needs special permissions. Only system user and administrators group must have access to it.

To remove other users from accessing this file, you may need to 

Right click administrators_authorized_keys , go to Properties -> Security -> Advanced

Click "Disable inheritance";

Choose "Convert inherited permissions into explicit permissions on this object" when prompted;