Unable to connect to backend http error-0

Hello,

I am seeing the error “Unable to connect to backend. HTTP error 0”. I read this tread:
https://wordpress.org/support/topic/unable-to-connect-to-backend-http-error-0/

I do see “var ajaxurl = ‘/wp-admin/admin-ajax.php’,” in the code,

The page is requesed by this url:

/wp-admin/admin.php?page=file_manager_advanced_ui

Do you have any suggestions on how I can fix this? Thanks.

Jeff