Persistent 'Error Adding Trello to Slack' After Full Reset

Question

**Details:**

I'm unable to link my Trello and Slack accounts. Every attempt fails with the same error page:

> Uh oh! There was an error adding Trello to Slack. Please try again in Trello.

I have three distinct error IDs from three separate attempts, confirming this isn't a one-off:
- `5dfdaaa4-44ec-48d9-a274-f9671e8c8b38`
- `a5fd7708-9353-44a1-ac8e-66654e897706`
- `836dfd24-2d0d-4d60-b80e-6c06236b846d`

**Steps already completed:**
1. Revoked the Trello Slack App token at trello.com/my/account → Applications → Revoke
2. Ran `/trello logout` then `/trello login` in Slack
3. Attempted re-authorization — same error
4. Retried in Chrome Incognito with no extensions — same error

**Environment:**
- Browser: Chrome (latest), Incognito mode
- OS: macOS
- Trello plan: Free
- Slack: installed and approved by workspace admin

The fact that each attempt generates a new unique error ID suggests the OAuth flow is initiating but failing on the backend, not a client-side issue. This is not resolved by the standard troubleshooting steps in the support docs.

Has anyone seen this resolved? Is there a known backend issue with the Slack auth flow currently?

D-Ramadhani · Answer

@Rena I'm having similar issues. I tried all the steps that Jonathan did and followed your steps too. None of them work.

It may be that I initially had to merge two trello accounts but it shouldn't be a problem anymore I thought since I successfully merged them.

Rena_854uq2 · Answer

Hi, thanks for writing in and for trying basic troubleshooting steps already! Engineer on Trello here 👋

The OAuth connection between Trello and Slack relies on your browser preserving a session cookie during the authorization process. If anything interferes with this (for example, third-party cookie restrictions, VPNs, or security software), the connection can fail with the error you’re seeing.

Can you try these additional steps?

* In Chrome, go to Settings → Privacy → Cookies, and temporarily allow all cookies (or specifically allow third-party cookies for Trello and Slack).
* Try the connection in a different browser (like Firefox or Edge), or on another device.
* If you use a VPN or are on a corporate network, try connecting on a different network or with the VPN disabled.

If these steps aren't resolving the issue, let us know so we can investigate further.

Jonathan Hess_4mptd8 · Answer

I was able to 'add' slack from the trello workspace but that still does not do the job. it shows up as connected in the trello workspace, and shows up as connected in the slackbot's 'message' tab, but does NOT show up in the slackbot's 'home' tab so any time i actually try to use it, it prompts me to log in again in the broken flow above ^.

I don't know why this is such a miss.