After uploading some pics, I selected "Export Metadata" in "Bulk actions", clicked "Check all" and then "Apply".
I have following entry in the debug.log
{8686:Wed, 31 Jan 2018 12:15:56 GMT}
WARNING: call_user_func() expects parameter 1 to be a valid callback, function 'xmpMetadataPublish' not found or invalid function name in /var/www/www.example.com/zenphoto/zp-core/admin-functions.php on line 4092
call_user_func called from processImageBulkActions (admin-functions.php [4092])
from admin-edit.php [269]
{8686:Wed, 31 Jan 2018 12:15:56 GMT}
WARNING: call_user_func() expects parameter 1 to be a valid callback, no array or string given in /var/www/www.example.com/zenphoto/zp-core/admin-functions.php on line 4092
call_user_func called from processImageBulkActions (admin-functions.php [4092])
from admin-edit.php [269]
{8686:Wed, 31 Jan 2018 12:15:56 GMT}
WARNING: call_user_func() expects parameter 1 to be a valid callback, no array or string given in /var/www/www.example.com/zenphoto/zp-core/admin-functions.php on line 4092
call_user_func called from processImageBulkActions (admin-functions.php [4092])
from admin-edit.php [269]
{8686:Wed, 31 Jan 2018 12:15:56 GMT}
WARNING: Cannot modify header information - headers already sent by (output started at /var/www/www.example.com/zenphoto/zp-core/admin-functions.php:4092) in /var/www/www.example.com/zenphoto/zp-core/admin-edit.php on line 347
header called from admin-edit.php [347]