Update to 3.5 disables the visual editor for posts and pages

First of all, you must know, that I was searching a lot through different forums to find a solution to my problem: I have updated from 3.4.2 to 3.5 and the editor was broken. With a fresh 3.5 installation, all is fine, but in my runing site, the behaviour is as follows:

The “Visual” mode has no toolbar at all !!!

If I want to edit (or create) a post or page and the “Visual” mode is already on, the “visibility: hidden” of the texarea wp-editor-area doesn’t allow me to see anything previously written or to set the cursor in that area. Even if I change to “Text” mode, the style “visibility: hidden” remains hard coded (no css!) Here the HTML code there:

textarea id=”content” class=”wp-editor-area” name=”content” cols=”40″ style=”height: 360px; resize: none; visibility: hidden;”

If I try to edit a post or page and the “Text” mode is already on, then the visibility is not hidden. Here the HTML code now:

textarea id=”content” class=”wp-editor-area” name=”content” cols=”40″ style=”height: 360px; resize: none;

Of course, now I can edit HTML (text), but as soon as I change to “Visual” and back to “Text“, the visibility is gone (hidden) again.

I have compared exactly this HTML code with the fresh (working) installation, and there I’ve found:

textarea id=”content” class=”wp-editor-area” name=”content” cols=”40″ style=”height: 360px; resize: none; display: none;” aria-hidden=”true”

And an aditional span can be found here (it’s missing in the buggy update):

span id=”content_parent” class=”mceEditor wp_themeSkin” role=”application” aria-labelledby=”content_voice”

It seems that this problem appears to others too and other forums describes different causes for that. I’ve tried them all w/o success. 🙁 May be can someone help me with a clue – what can I try or how can I further search the cause. This is what I’ve tried w/o success:

1. I’ve disabled ALL plugins – but, because the visibility style is hard coded, I actually think that there is a problem in WP 3.5, and not in the plugins.

2. I’ve added in wp-config.php – as some forums suggested:

define(‘SCRIPT_DEBUG’, true );
define(‘CONCATENATE_SCRIPTS’, false );

3. I’ve changed (took from the fresh installation) the directory:

wp-include/js/tinymce

I hope my own investigation will help some WP developer to find out the cause and to publish the solution.

Thanks in advance
snookersky