Page disrupted after update

Hi, I have recently updated my website by installing a SSL certificate. Then I updated to PHP 7.3 (from 6.5) because some features/plugin didn't work anymore, althought after doing that I noticed that backend page updating failed every time by giving back a critical error.
I fixed that issue too by editing a function inside the theme.
Now a new issue appears.
When I try to update a page I find the same page in backend area completely upset, with the whole builder empty.
What can I do? 
I have installed WordPress 5.5.3

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