thank you for the link. I found out why the page is blank, and I think a good solution would be to convert a null result to an empty array. So php (5.6.14) won’t stop processing but simply skip the loop.
On another server (php 5.6.29), the error does not occur.
Error log: PHP Warning: Invalid argument supplied for foreach() in /…/lib/pkp/classes/submission/form/PKPSubmissionSubmitStep1Form.inc.php on line 36