OAuth not working

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Steps to reproduce

1. Install the integration
2. Create Dropbox app with 3 requested read permissions
3. Enter key and secret in Nextcloud settings

Expected behaviour

Data Migration in User Settings should allow import of files from Dropbox

Actual behaviour

Clicking on Data Migration throws an error message: Failed to get Dropbox storage information
Log entry: Dropbox OAuth error : Client error: 'POST https://api.dropboxapi.com/oauth2/token' resulted in a '400 Bad Request' response: {"error": "invalid_grant", "error_description": "refresh token is invalid or revoked"}

Server configuration

Web server: Nginx

Database: MariaDB

PHP version: 8.2.27

Nextcloud version: Nextcloud Hub 9 (30.0.4)

Start with the Data Migration flow in User Settings and the Dropbox OAuth token request reported in the log. Reproduce the 400 invalid_grant response with the stated Nextcloud, PHP, and Dropbox-app setup, then trace where the integration handles the token failure. Done means Dropbox file import works without the reported error.