My problem is exactly the same as describe in this old issue  MDL-46500, but with moodle version 4.4.1

The javascript error is : "filemanager is null"

In files/renderer.php, function fm_print_generallayout is called twice (for file1 and file2), generating two client_id

In the rendered html file, only one filemanager is present, so the code at the end of lib/form/filemanager.js :

var filemanager = Y.one('#filemanager-'+options.client_id);

stop on error "filemanager is null" for the filemanager for file1 which is not include in the template.