4.9.8 fr Weird behaviour, no media, many buttons unresponsive,

Since a few days, I have incredible problems with a one WP blog. Long story short, a few examples: the media shows an empty page, but I can upload files, albeit with a strange interface, like old-style, and images are indeed uploaded.

In the pages/posts edit interface, I can't switch between visual and text, and I can't do anything with images - the buttons are like dead. If I make some changes in a post or page and switch to another page, WP doesn't reminds me to save my works, as if I haven't done anything. In the plugins' list, I can't select all extensions, I click the checkbox above the list and nothing happens. Everything is up-to-date.

So far, I have (unsuccessfully) tried to disable all plugins and my theme, up the memory to 256M, up the php from 7.0 to 7.2 and made a complete manual update via FTP (twice!).

It is on a dedicated server, with half a dozen other such WP blogs that don't show any problem.

Any clue?