@OctavianC your code will not work, let me explain:
tinymce display needs an id:
(plugins/editors/tinymce/tinymce.php:121)

    public function onDisplay($name, $content, $width, $height, $col, $row, $buttons = true, $id = null, $asset = null, $author = null)

If non is given then this code will be executed:
(plugins/editors/tinymce/tinymce.php:123)

        if (empty($id))
        {
            $id = $name;
        }

and obviously an id with brackets won't be accepted

Worked before. Also, the $id is optional so why not clean it before using it? It shouldn't take more than a couple of lines.

There was a change in tinMCE (joomla side): before the selector for tinMCE was a class .mce-editable and now is using the id, check plugins/editors/tinymce/tinymce.php line 834

This is tinyMCE limitation http://wordpress.stackexchange.com/questions/156309/wp-editor-not-displaying-tinymce-correctly-when-square-brackets-used-in-name

So, if code worked correctly on previous versions of Joomla!, then this is a B/C break. There's obviously a small oversight, because there is some filtering done to the $id but that variable never gets used.

The issue is NOT with TinyMCE, but with its initialization, because HTML ids are not sanitized. It's a small fix which I'll provide in a little while.

NO! the code before was crap for multiple instances of tinyMCE in the same page, that was fixed with this change

I just love endless discussions without any arguments :)

try this:

        if (empty($id))
        {
            $id = $name;
            $id = str_replace('[', '__', $name);
            $id = str_repeat(']', '', $id);
        }

Closed as we have a PR for testing