SMTP notification doesn't find attached snapshot

I am trying to send an e-mail notification with a snapshot attached when frigate detects motion in certain areas using the great template by SgtBatten https://community.home-assistant.io/t/frigate-mobile-app-notifications-2-0/559732
The mail arrives, but there is no snapshot attached. In order to figure out the basics I tried a simple script, following also this post here https://community.home-assistant.io/t/attaching-images-to-email-notifications/217291/5

 alias: 01_Test_SMTP
    sequence:
      - service: notify.webde
        data:
          title: HA Testalarm
          message: Attachment Test
          data:
            images:
              - /api/frigate/notifications/1701857892.726853-la20uy/snapshot.jpg
    mode: single
    icon: mdi:mail

Also with this script, the mail will arrive, but without the attachment. The protocol file says

Attachment /api/frigate/notifications/1701857892.726853-la20uy/snapshot.jpg not found. Skipping

However, when I use a picture card with the exact same link, the snapshots pops up immediately - so the file is there and accessible. Btw, I tried using single or double quotes for the link with no success.

What am I missing here? Does anybody have any idea

Thanks a lot for your help
Klaus