Internal links and media manager are inoperative after upgrade to WP 5.5

Hello,

One of my clients is still using the classic editor on her WordPress. Since the upgrade to 5.5 she’s having at least 2 serious problems:

1. No “internal” links on the edit page works (insert media, change publication date, switch to html editor …). She clicks and nothing happens.

2. Imposible to insert featured images. The media manager no longer opens in a pop-up window. It opens in the main window. After uploading the image and indicating that it be used as a featured image… nothing happens. The page does not redirect, there’s no close button, nothing. If she selects insert into input, even worse. Blank page.

How can these two problems be solved? Right now this person has her website totally inoperative.

Thanks in advance.