# Slack URL Verification: “Workflow Webhook Error: Workflow could not be started!” despite Active workflow (Self-hosted, proxy)

**URL:** <https://community.n8n.io/t/slack-url-verification-workflow-webhook-error-workflow-could-not-be-started-despite-active-workflow-self-hosted-proxy/212735>\
**Category:** Questions\
**Created:** [October 28, 2025, 7:12am UTC](https://community.n8n.io/t/slack-url-verification-workflow-webhook-error-workflow-could-not-be-started-despite-active-workflow-self-hosted-proxy/212735 "2025-10-28T07:12:17Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![SaucyJack](https://avatars.discourse-cdn.com/v4/letter/s/a4c791/32.png) [@SaucyJack](https://community.n8n.io/u/SaucyJack)\
**Post date:** [October 28, 2025, 7:12am UTC](https://community.n8n.io/t/slack-url-verification-workflow-webhook-error-workflow-could-not-be-started-despite-active-workflow-self-hosted-proxy/212735/1 "2025-10-28T07:12:18Z")

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Hi all, I need help with Slack Events API verification.  
Environment  
• n8n: 1.116.2 (Self-Hosted)  
• Docker + Postgres backend  
• Reverse Proxy: Caddy v2.10.2  
• Host: Ubuntu 22.04 LTS  
• WEBHOOK\_URL + N8N\_HOST + N8N\_PROTOCOL set correctly  
• N8N\_TRUST\_PROXY=true  
Slack Setup  
• Request URL:  
[https://n8n.hansei.pro/webhook/slack-events](https://n8n.hansei.pro/webhook/slack-events)  
• Event: url\_verification  
• Method: POST  
Workflow  
• Webhook node (POST, respond via Respond to Webhook)  
• IF → Respond to Webhook (text body = challenge string)  
• Workflow is Active with green dot  
Symptoms  
• GET returns correct 404: webhook not registered for GET  
• Webhook is reachable via browser  
• But POST returns:  
{“code”:0,“message”:“Workflow Webhook Error: Workflow could not be started!”}  
• Same result using curl test payload:  
-d ‘{“type”:“url\_verification”,“challenge”:“abc123”}’  
• No webhook-related logs appear in docker logs n8n  
Goal  
Slack URL Verification should return raw challenge string (e.g. “abc123”) and show ✅ Verified in Slack.  
What I suspect  
Webhook endpoint is not fully registered in the execution engine. Maybe something with proxy headers / webhook registry.  
Any insight appreciated. Thank you! find solutions is by using the 🔎 search function at the upper right.  
If your question hasn’t been asked before, please follow the template below. Skip the questions that are not relevant to you. →

## Describe the problem/error/question

## What is the error message (if any)?

## Please share your workflow

```auto
(Select the nodes on your canvas and use the keyboard shortcuts CMD+C/CTRL+C and CMD+V/CTRL+V to copy and paste the workflow.)

```

## Share the output returned by the last node

## Information on your n8n setup

- **n8n version:**
- **Database (default: SQLite):**
- **n8n EXECUTIONS\_PROCESS setting (default: own, main):**
- **Running n8n via (Docker, npm, n8n cloud, desktop app):**
- **Operating system:**

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<div class="post-metadata">

**Author:** ![barn4k](https://dub1.discourse-cdn.com/n8n/user_avatar/community.n8n.io/barn4k/32/8170_2.png) [@barn4k](https://community.n8n.io/u/barn4k)\
**Post date:** [October 29, 2025, 10:45am UTC](https://community.n8n.io/t/slack-url-verification-workflow-webhook-error-workflow-could-not-be-started-despite-active-workflow-self-hosted-proxy/212735/2 "2025-10-29T10:45:13Z")

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hello @SaucyJack

Please, share the workflow.

You can select all nodes with **Ctrl+A** and copy them with **Ctrl+C**. Then, past the content after pressing the button `</>` with **Ctrl+V**.

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**Author:** ![system](https://dub1.discourse-cdn.com/n8n/user_avatar/community.n8n.io/system/32/195824_2.png) [@system](https://community.n8n.io/u/system)\
**Post date:** [January 27, 2026, 10:45am UTC](https://community.n8n.io/t/slack-url-verification-workflow-webhook-error-workflow-could-not-be-started-despite-active-workflow-self-hosted-proxy/212735/3 "2026-01-27T10:45:14Z")

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Closed on January 27, 2026, 10:45am UTC

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