can't update any pages from the dashborad

Hi,
I use ninja form plugin. everything was working fine until few days ago, where I started to receive mails from people that cannot submit the form that they fill out: after few second a white page appear and nothing happen. I realize then that I cannot update any pages from the Dashbord, same thing: when I click UPDATE button on a page, after few second, white page and nothing has been updated.

I tried to desactivate plugins, to see if the problem come from a plugin’s update, and I cannot even desactivate any plugin, same thing happen: white page.

You can try the form and click submit button to see what is happening, same thing is happening when I want to update a page from the dashbord:
https://passionofheart.com/programs/women-and-passion/womens-heart-gathering/whg-application-form/

Do you have any suggestion to help me resolve this problem?
Thanks,