"Appearence>Customize" screen does not load properly

Hi!
WP 4.3 gives me troubles with the Appearance>Customize section. When I go there I just can see a small part of the content in the left sidebar (just showing site name and the theme I’m previewing. Main screen shows the gear as if it’s loading but it steys there and nothing happens.
With Chrome I see this:
Uncaught TypeError: Cannot read property ‘get’ of undefined load-scripts.php:29

With Firefox
TypeError: i is undefined load-scripts.php:29:28892
downloadable font: rejected by sanitizer (font-family: “dashicons” style:normal weight:normal stretch:normal src index:0) source: https://www.redclash.com/wp-includes/fonts/dashicons.eot load-styles.php:1:11

I guess is a problem of the 4.3 version.
How to solve that?
Can I re-install (and how)?
Is there a way to get back to previous version?

Any help welcomed