“Fatal error” when saving/publishing

Hi! I really like your plugin, and I have used it to publish the 1st edition of a magazine. Now I want to publish the 2nd edition, and I can create a new book and even save it, but after I have picked the PDF and thumbnail files and click on “save” (or “publish”) I always get a fatal error:

Fatal error: Uncaught TypeError: array_merge(): Argument #1 must be of type array, string given in /home/…./wp-content/plugins/3d-flipbook-dflip-lite/inc/metaboxes.php:705 Stack trace: #0 /home/…./wp-content/plugins/3d-flipbook-dflip-lite/inc/metaboxes.php(705): array_merge(‘a:28:{s:11:”sou…’, Array) #1 /home/…./wp-includes/class-wp-hook.php(326): DFlip_Meta_boxes->save_meta_boxes(2212, Object(WP_Post)) #2 /home/…./wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters(NULL, Array) #3 /home/…./wp-includes/plugin.php(517): WP_Hook->do_action(Array) #4 /home/…./wp-includes/post.php(4828): do_action(‘save_post’, 2212, Object(WP_Post), true) #5 /home/…./wp-includes/post.php(4930): wp_insert_post(Array, false, true) #6 /home/…./wp-admin/includes/post.php(453): wp_update_post(Array) #7 /home/…./wp-admin/post.php(227): edit_post() #8 {main} thrown in /home/…./wp-content/plugins/3d-flipbook-dflip-lite/inc/metaboxes.php on line 705

Do you have any idea what could be the cause of this? Everything on the site is up to date (theme: Enfold).

Thanks!

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