Error with User Section in Debug Toolbar: Header already sent


I’m wondering about an Error i get in the Debug Tool Bar (User section, on the right). When I’m not logged in my Application, there is written “Guest” but when i’m logged in: error:

session_set_cookie_params(): Cannot change session cookie parameters when headers already sent

The Error occures the first time, my Application is calling:
if (empty(Yii::$app->session['shopmodule_multilanguage']))

How can i debug this further? My Application is not throwing a Notice or an Error. It only shows up in Debug Toolbar.

I use PHP7.3.7 and Yii 2.0.30 and Debug 2.0.14