This Writer-> save error may be due to many reasons, part of which is due to errors caused header and buffer.
This particular part of the discussion can be seen here http://stackoverflow.com/questions/8566196/phpexcel-to-download
However, there is a question not easy to find, at high version PHP7, ERR_INVALID_RESPONSE of error may also be due to the following reasons.
Fatal error: 'break' not in the 'loop' or 'switch' context in <mypath> \ PHPExcel \ PHPExcel \ Calculation \ Functions.php on line 581 Open PHPExcel \ Calculation \ Functions.php file, delete the line 581 can break
Quote Reference: https://github.com/PHPOffice/PHPExcel/issues/716