Wikimedia Bugzilla is closed!

I have found only 1 call to WebResponse::header with a potencial string as third parameter in core, that is in ApiMain line 386
$response->header( $headerStr, true, $e->getCode() );

$e is a expection, where getCode is documented with a return type of int, but
http://www.php.net/manual/en/exception.getcode.php says:
Returns the exception code as integer in Exception but possibly as other type in Exception descendants

A stack trace is needed to find out, if this is from api (and which exception type) or maybe a extension.