Classloading problem: Error on save of custom type

Hi, I get this issue when I save a custom post type that is associated with a field group.
The field group is displayed no problem when I edit the custom post, but then when I want to save, this is the error message, it seems to be a classloader issue:
PHP Fatal error: Uncaught Error: Class ‘MBB\Attribute’ not found in /Users/xxxxxx/Documents/src/xxxx.xx/wp-content/plugins/meta-box-builder/bootstrap.php on line 16
Is this a known issue? what to do?