Page content drops out when updating the page

Hello,
We’ve updated our theme, WordPress and all plugins to the latest version, but still have problems updating certain pages. When we make a change to the page and then update the page, most of the content drops out. The page with the problem is long and has a lot of elements on there, but there are other pages on our site which are just as long and have just as many elements which don’t have this problem.

Please help! Thank you.

Jo