I don't know if it is Sellacious that is not compatible with Fabrik, or the other way around, but I need to use both software.
I did this test with Sellacious 1.7.3 and with Sellacious 2.0 Alfa2, and the problem is the same.
After installing Fabrik, this error appears in frontend and backend:
Fatal error: Cannot declare class Joomla\CMS\Form\FormField, because the name is already in use in my_root/administrator/components/com_fabrik/classes/39/FormField.php on line 22
Do you have any idea what the problem might be?