I've had this happen to me before, and it was because I had some faulty TinyMCE configuration javascript (in the dashboard "Rich Text Editor" page). Do you have a custom TinyMCE configuration on your site? Or are you just using one of the pre-built ones ("Simple", "Office", etc.)?

Also, check your Web Inspector or Firebug for javascript errors -- that might also indicate what the problem is.

Dear Jordan,
Thank You very much for your reply.

I use the default configuration. But I solved the problem a few hours ago by changing my designer-content-block.

The toolbars are visible and editable since I took a "textbox" instead of a "image" before the "WYSIWYG-Editor". (see file)

Now I have to insert a filename instead of choosing a pic. This is even better, because I don't have to upload over 700 images into the file manager.

I hope, You can use this information to find out, what went wrong.