BE "file_exists() / restriction in effect" error => encoded "ü" in path

Hi, I'm getting an Uncaught Exception error when opening the extension settings. (> screenshot)

It works locally (different domain), I'm guessing it has something to do with the encoded "ü" in the path.

Example: http://ätest.com => http://xn--test-koa.com

PS. fresh install, server setup is identical to our others where we had no problems

# path on server

$ ls
Chart.js  Chart.min.css  Chart.min.js  chartjs_2.94.min.js  datalabels.min.js
$ pwd
/web/current/public/typo3conf/ext/sg_cookie_optin/Resources/Public/JavaScript/Backend/Chart.js

# BE error

Core: Exception handler (WEB): Uncaught TYPO3 Exception: #1476107295: PHP Warning: file_exists(): open_basedir restriction in effect. File(/typo3conf/ext/sg_cookie_optin/Resources/Public/JavaScript/Backend/Chart.js/Chart.min.css) is not within the allowed path(s): (/var/www/clients/client1/web219/web:/var/www/clients/client1/web219/private:/var/www/clients/client1/web219/tmp:/var/www/xn--asdf-t9a.ch/web:/usr/share/php5:/usr/bin:/usr/share/php:/tmp:/etc/ssl/certs:/dev/urandom) in /var/www/clients/client1/web219/web/releases/6/public/typo3/sysext/core/Classes/Utility/GeneralUtility.php line 2309 | TYPO3\CMS\Core\Error\Exception thrown in file /var/www/clients/client1/web219/web/releases/6/public/typo3/sysext/core/Classes/Error/ErrorHandler.php in line 137. Requested URL: https://www.xn--asdf-t9a.ch/typo3/module/web/SgCookieOptinOptin?token=--AnonymizedToken--