Custom error page in v1.7

I defined static configuration:

[backends]

  [backends.dummy]
    [backends.dummy.servers]
      [backends.dummy.servers.server0]
		url = "https://www.google.com"
  [backends.errors]
    [backends.errors.servers]
      [backends.errors.servers.server0]
		url = "https://error"

and my service is defined like:

services:
  funding:
     labels:
        - "traefik.frontend.errors.errpage.backend=errors"
        - "traefik.frontend.errors.errpage.status=401"
        - "traefik.frontend.errors.errpage.query=/401.html"

The funding is an api service. When I make a GET request to the funding and got 401, but the error page doesn't show.

What did I miss?

I found that when I deploy funding service, the traefik log show:

Error when creating error page "networks" for frontend "frontend-web-dummy-funding-web": the backend "backend-errors" doesn't exist.","time":"2020-09-16T11:05:03Z"}.

But in traefik UI I saw the file provider has the backend errors

File and Docker provider cannot live together in v1.7?

Hope somebody can help.

Got stuck 5 days.