Setting SLACK_WEBHOOK_URL from organization secret breaks build

[armNH]

Setting SLACK_WEBHOOK_URL from organization secrets leads to faild build with message: An HTTP protocol error occurred: statusCode = 302

Log:
Run 8398a7/action-slack@v3
with:
status: custom
fields: workflow,job,commit,repo,ref,author,took
custom_payload: {
attachments: [{
title: :construction: BUILDING: ${process.env.AS_WORKFLOW},
color: 'warning',
text: ${process.env.AS_JOB} (${process.env.AS_COMMIT}) of ${process.env.AS_REPO}@${process.env.AS_REF} by ${process.env.AS_AUTHOR},
}]
}

author_name: 8398a7@action-slack
github_token: ***

env:
SLACK_WEBHOOK_URL: ***
Error: An HTTP protocol error occurred: statusCode = 302

Workflow command:

• name: Notify BUILD START to Slack
  uses: 8398a7/action-slack@v3
  with:
  status: custom
  fields: workflow,job,commit,repo,ref,author,took
  custom_payload: |
  {
  attachments: [{
  title: :construction: BUILDING: ${process.env.AS_WORKFLOW},
  color: 'warning',
  text: ${process.env.AS_JOB} (${process.env.AS_COMMIT}) of ${process.env.AS_REPO}@${process.env.AS_REF} by ${process.env.AS_AUTHOR},
  }]
  }
  env:
  SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK}}
  if: always()

If i set the same secret on the repository than it works as expected and there is no error.