Steps to reproduce

0. Enable dropbox app. (release/1.0.1).
1. Having an external storage available in dropbox.com. Set it up in the storage section of admin settings.

Expected behaviour

Click in "grant access" button redirects to dropbox site to allow access.

Actual behaviour

Grant access button doesn't redirect anywhere.

When completing the form we can see a POST request failing:

Request URL: HOST/index.php/apps/files_external/globalstorages
Request Method:POST
Status Code:422 Unprocessable Entity

Response:
{"message":"Unsatisfied authentication mechanism parameters"}

Server configuration

Operating system:
Ubuntu 16.04

Web server:
Apache

Database:
MySQL

PHP version:
7.1

Cache
DB

ownCloud version: (see ownCloud admin page)
{"installed":true,"maintenance":false,"needsDbUpgrade":false,"version":"10.0.4.4","versionstring":"10.0.4","edition":"Community","productname":"ownCloud"}

Updated from an older ownCloud or fresh install:
Fresh

The content of config/config.php:

Are you using external storage, if yes which one: local/smb/sftp/...
Yes, dropbox.

Are you using encryption:
No.

Logs
Nothing appears in the logs as warnings or errors.

Client configuration

Browser
Chrome