Blank pages when I update a page (after updating the theme)

Dear Support,

After updating the theme, any time I update a page, the page goes blank and all content is removed. I have tried adding all different types of content (text, images, buttons) and I have enabled the WP Classic Editor in Theme Options, but nothing has worked.

I have been able to restore previous versions of pages, but need to add new content. Any information on how this can be fixed will be greatly appreciated. Thanks.