...Most likely the email was intercepted at his end somehow.
Yes, it was. I am solving the problem.
Who is your email provider, maybe one of their staff intercepted it?
Also, just because an IP isn't in an email security log doesn't always mean it wasn't accessed by it. Mistakes can happen by the mail servers.
You're quite lucky your account was recoverable so quickly.
The problem is in that security of my PC has been compramized.