Upgrade from Perch 2.8 to Runway - backup error

I just upgraded a page from Perch 2.8.x to the newest Runway, and everything went smooth with that process,

but as soon as I added buckets.php and the access token to my runway.php file, the /core/settings/backup/ just shows a 500 HTTP Error.

This is the current code in the buckets.php

<?php
    return [
        'backup' => [
                 'type'      => 'dropbox',
                 'web_path'  => '',
                 'file_path' => 'backups',
         ],
    ];

and this is the code in the runway.php section:

<?php
    'dropbox' => [
            'access_token' => 'f_Xwdju..........................',
            'handler'      => 'PerchDropbox_ResourceBucket',
            'handler_path' => PERCH_CORE.'/runway/apps/perch_dropbox/PerchDropbox_ResourceBucket.class.php',
        ],      

Have done this process on multiple pages before, but usually coming from Perch 3 before the upgrade to Runway, but I don't know if that would be a reason?

Notable mentions would be that the "old" Perch Backup app previously was installed, and still have some database tables as far as I can see. (but the app has now been deleted from server)

Diagnostics:

Perch Runway: 3.1.1, PHP: 5.6.36-0+deb8u1, MySQL: mysqlnd 5.0.11-dev - 20120503 - $Id: 76b08b24596e12d4553bd41fc93cccd5bac2fe7a $, with PDO
Server OS: Linux, cgi-fcgi
Installed apps: content (3.1.1), categories (3.1.1), assets (3.1.1)
App runtimes: <?php $apps_list = array( );
Image manipulation: GD Imagick
PHP limits: Max upload 128M, Max POST 128M, Memory: 129M, Total max file upload: 128M
Resource folder writeable: Yes

If I remove the code from the buckets.php file, the backup page works again, but I can ofcourse then not select any bucket.

Any thoughts?