admin only in html unless in debug mode

I recently used some of the cpanel security tools to improve security, as recommended in the WordPress toolkit. But that gave a critical error and I had to restore the site from a backup (using cpanel restore tools).

But after that, the backend (admin section) shows in html only and is unusable. The website front end seems to works fine.

Only when I put the site in debug mode with the option to use the core (non-minified) versions of core CSS and JavaScript files the admin section works again. But this is of course having impact on performance and not normal. I want to find the cause and solve this.

I tried to reinstall WP core 6.1.1, tried to deactivate all plugins, updated everything possible, no success. I also checked all file permissions they are as they were.

Anyone?

The page I need help with: [log in to see the link]