This question has been answered by frogabog. See the first response.
In system settings, turn off compress_css and compress_js. If you can't get there via the manager, do this in the site_settings table, change both from 1 to 0 and clear the files in the core/cache folder.
This by itself probably won't fix it (but it might). Get a fresh copy of MODX.zip, upload to the server, extract the files (on the server, no FTP), and re-run setup as an upgrade.