Failure in Page composer – "Loading; Please wait…"

Hi,
I have some issues with our website where I can’t modify the Pages anymore from the wp-admin account. When I “Edit Page”, all components are displayed in HTML code in the “classic editor” mode. When I switch to the “Page composer” mode with the blue button, only first page section is displayed and there is no more scrolling bar on the right side of the page to navigate between the sections and elements.
I’m currently working with the 4.3.1 version of WP. I saw on forums that these problem was reported on past version; How can I fixe it in the current version?
Thanks in advance