I found the cause, there is a fatal error in the log related to LESS:
PHP Fatal error: Uncaught exception ‘Less_Exception_Parser’ with message ‘File
styles/index.lessnot found.’ in /var/www/html/ojs/lib/pkp/lib/vendor/oyejorge/less.php/lib/Less/Parser.php:2606
I’m testing this in a fresh install from a tarball downloaded from OJS download site, so it should not be related to the problem with not installed dependencies as I had in the beginning with the installation from git.
I found a similar error here: CSS and Java Script not loaded in OJS 3.0.2 backend - #4 by Vitaliy, although in my case the problem is only with CSS loading.
I inspected the network communication using Chrome, and there are two files that are not loaded in the backend:
/ojs/styles/build.css shows a 404 error
/ojs/index.php/IAS/$$$call$$$/page/page/css shows a 500 error
Regards,
Ales