Nextcloud Mail not working with Strato Mail

Dear Community,

I am trying to solve this one for a long time and my Strato Support is not giving answers :unamused:. In the following you should find every intel I can provide and

Nextcloud version: 11.0.3
Operating system and version : Raspbian Jessie 8.0
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.10
PHP version (eg, 5.6): PHP 5.6.30-0+deb8u1
Is this the first time you’ve seen this error?: About half a year ago after changing to a raspberry 3 as a hosting maschine

Can you reliably replicate it? (If so, please outline steps):

  1. Create a nextcloud instance with mail
  2. Try to register a Mail account with a Strato mail hosting.
  3. Creating acc failed. "Server denied authentication
    The issue you are facing:
    Cannot create a mail account with strato as a host.

The output of your Nextcloud log in Admin > Logging:
Error mail Creating account failed: Server denied authentication. 2017-05-18T14:43:08+0200
Error PHP Undefined offset: 1 at /var/www/nextcloud/apps/mail/vendor/ 2017-05-18T14:43:06+0200

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
See here:
The output of your Apache/nginx/system log in /var/log/____:
Will update soon from a computer with server access.