After some investigating looks like what is happening is both WPBook and Custom Admin Branding are using a function named “custom_header()” and “custom_footer”. In WPBook 1.5 this function has been renamed but for now the easiest fix should be changing the function name inside of custom_branding.php
Try changing line 501 from function custom_header()
to function custom_header_cab() {
and then update line 568 from add_action('admin_head', 'custom_header', 11);
to add_action('admin_head', 'custom_header_cab', 11);
You’ll have to do the same thing for custom_footer() on line 572 from function custom_footer() {
to function custom_footer_cab() {
then on line 607 from add_action('admin_footer', 'custom_footer');
to add_action('admin_footer', 'custom_footer_cab');
Keep in mind you can use any name you want for the function as long as it’s not used by another plugin. You also could change the function name being use in the wpbook/theme/config.php and then update the wpbook/theme/index.php I just thought it may be easier to update the other CAB plugin.
Let me know if that works