Problem with www subdomain

Describe the issue or problem
Calling via domain.com is no problem. But as soon as I call the website via www.domain.com the following error occurs. I have no idea what could be the reason for this.

Steps I took leading up to the issue
Since installation.

What application are you using?
For example, 3.3.0.11

Additional information
Warning: ini_set(): A session is active. You cannot change the session module’s ini settings at this time in /is/htdocs/xxx/www/lib/pkp/classes/session/SessionManager.inc.php on line 69

Warning: Cannot modify header information - headers already sent by (output started at /is/htdocs/xxx/www/lib/pkp/classes/session/SessionManager.inc.php:69) in /is/htdocs/xxx/www/lib/pkp/classes/session/SessionManager.inc.php on line 223

Warning: Cannot modify header information - headers already sent by (output started at /is/htdocs/xxx/www/lib/pkp/classes/session/SessionManager.inc.php:69) in /is/htdocs/xxx/www/lib/pkp/classes/session/SessionManager.inc.php on line 231
400 Bad Request