Invalid Argument acf-functions.php

Hi, just added your Plugin to a new Multisite. When I try to duplicate a page (or post), I always get this error message:

Warning: Invalid argument supplied for foreach() in /nhldata/749/156749/data/web/web01/htdocs/wp-content/plugins/multisite-post-duplicator/inc/acf-functions.php on line 80

Warning: Invalid argument supplied for foreach() in /nhldata/749/156749/data/web/web01/htdocs/wp-content/plugins/multisite-post-duplicator/inc/acf-functions.php on line 179

Warning: Invalid argument supplied for foreach() in /nhldata/749/156749/data/web/web01/htdocs/wp-content/plugins/multisite-post-duplicator/inc/acf-functions.php on line 80

Warning: Invalid argument supplied for foreach() in /nhldata/749/156749/data/web/web01/htdocs/wp-content/plugins/multisite-post-duplicator/inc/acf-functions.php on line 179

Warning: Cannot modify header information – headers already sent by (output started at /nhldata/749/156749/data/web/web01/htdocs/wp-content/plugins/multisite-post-duplicator/inc/acf-functions.php:80) in /nhldata/749/156749/data/web/web01/htdocs/wp-admin/post.php on line 197

Warning: Cannot modify header information – headers already sent by (output started at /nhldata/749/156749/data/web/web01/htdocs/wp-content/plugins/multisite-post-duplicator/inc/acf-functions.php:80) in /nhldata/749/156749/data/web/web01/htdocs/wp-includes/pluggable.php on line 1179

The posts/pages will be duplicated and the linking seems to work too. But on every change (edit/delete) I always get this error. As long it’s working it wouldn’t be such a big problem for me, but our customer wanted it to be fixed. Do you have an idea what it could be, or how to resolve it?

We are using ACF Pro Fields for all Posts/Pages on the site…

Thank you in advance, Christian.