Additional Settings Error

Hi Guys.

When Im in the web browser and I click on additional settings the following error occurs (the error occurs only when im clicking on additional settings):

Sry for the german. It only says that an internal server error occured and that i have to contact the system administrator (which I am).
The nextcloud.log with the error code:

{"reqId":"Yakc9IjQ7EFpcMc2eQc4","level":3,"time":"2018-04-16T17:55:52+00:00","remoteAddr":"","user":"admin","app":"PHP","method":"GET","url":"\/index.php\/settings\/admin\/additional","message":"Cannot modify header information - headers already sent by (output started at \/var\/www\/html\/custom_apps\/passman\/personal.php:1) at \/var\/www\/html\/lib\/private\/legacy\/response.php#108","userAgent":"Mozilla\/5.0 (Windows NT 10.0; Win64; x64; rv:59.0) Gecko\/20100101 Firefox\/59.0","version":""}

I tried this from different hosts (Linux/Windows) and I get this error at 99% of the time. I know that it worked at least one time (because i used it for setting up the mail server).

Restarting the server does not help.
I´m using nextcloud in a docker with nginx-reverse-proxy.
Nextcloud 13.0.0
Ubuntu server 16.04.4
PHP 7.1.14

I hope that you can help me!