Original report: https://forum.mattermost.org/t/error-connecting-to-zapier/6410
Reproduced by JIRA reporter as well.
Steps to reproduce:
1. From the Mattermost System Console enable incoming webhooks, outgoing webhooks, custom slash commands, OAuth 2.0 service provider,
2. Then from the Integrations menu for my team, create a new OAuth 2.0 application for Zapier, following the directions at https://docs.mattermost.com/integrations/zapier.html#register-zapier-as-an-oauth-2-0-application, using callback URL https://zapier.com/dashboard/auth/oauth/return/MattermostDevAPI/
3. Next, at Zapier, create a new connected account for Mattermost, providing my Mattermost server URL with no trailing slash, the client ID and client secret copied from the Mattermost integration.
4. Click “Yes, continue,”
Observed: Error message from my mattermost server: “invalid_request: Supplied redirect_uri did not match registered callback_url”
Expected: Successful registration of the connected account.