Source

383                 $this->getStatusCode(),
384                 $this->getStatusDescription()
385             );
386             header($method ?? '');
387             foreach ($this->getHeaders() as $header => $value) {
388                 header("{$header}: {$value}", true, $this->getStatusCode() ?? 0);
389             }
390         } elseif ($this->getStatusCode() >= 300) {
391             // It's critical that these status codes are sent; we need to report a failure if not.
392             user_error(
393                 sprintf(
394                     "Couldn't set response type to %d because of output on line %s of %s",
395                     $this->getStatusCode(),
396                     $line,
397                     $file
398                 ),

Trace

• Couldn't set response type to 404 because of output on line 1 of /home/site/wwwroot/public/index.php
  HTTPResponse.php:392
• SilverStripe\Control\HTTPResponse->outputHeaders()
  HTTPResponse.php:345
• SilverStripe\Control\HTTPResponse->output()
  index.php:27