Visual Editor blank in WordPress 4.5 / .1

I have WP 4.5.1 now installed. Since the 4.5 update I am unable to use the Visual editor and the html tab would not even allow me to switch to text editing view. The visual editor shows white text on white.

I have switched to default theme, disabled all plugins, cleared cache. Same issue. It fails to work on multiple browsers.

This only began with 4.5 update.
It is unlikely a server issue as I have a dedicated server with several other instances of WP installed – and I was able to update them with no problems.

It seems to be a permissions issue, as the error console shows
ReferenceError: tinymce is not defined
GET https://myserver.com/folder/wp-includes/js/tinymce/wp-tinymce.php 403 Forbidden

BUT my folder permissions are 755 and files 644.

Enabling script_debug solves the issue, but is not a solution.
Rather desperate to resolve this as it is only a test site (thankfully) but I need to update a client’s site that has the same set up.