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]

Is the xmpMetadataPublish plugin enabled?

yes it is.
Only "3gp" and "mov" aren't marked.

Have to investigate. Admit this is one of the plugins I don't really use myself…

Generally please if you can please try the support build that is to become 1.4.15 sometime soon.

Off topic:
Which ZPC-builds are available?
I heard/have seen so far: the stable one I download from this page,
then the support build ( github? )
and zenphoto20 (github).

As I am a beginner I wanted to walk with "stable" - so I know, that - if some challenges appear - then I am the reason and not the APP

The officail build and the support build (master) from GitHub. Links are on our download page. That zp20 is a fork and separate project by a former team member we are not involved with. You are free to use that if it fits you but then you won't get support here ;-)