Imap with yahoo doesn't show up

I try to get a count of my new emails thru imap.

configuration.yaml:

sensor:
  - platform: imap
    name: emails
    server: !secret imap_server
    port: 993
    username: !secret imap_username
    password: !secret imap_password
    folder: Inbox

But the sensor.emails is not available thru developer tools.
I also searched this forum, but didn’t found what’s going wrong.
The Logfile has nothing about imap.
Is there a possibility to debug this any further? How? Any hints?

HomeAssistant: 1.12.3 thru VirtualBox VM