In addition, when clicking 'New' button, nothing happens.
After debugging in XAMMP/J4/PHP8/XDEBUG environment noticed that at some point in time variable $task has value "quickmode" instead of "new" after which incorrect function is called.
As I have a clean BF installation on J4/PHP8, initially it did not have any forms. I am able to import forms using packages created with J3/PHP8, however, none of these forms can be made visible, let alone edited as there is not default package selected. I guess I have to find out how to set this in the DB and link all forms to that one package.