Hi Holger,

You have a lots of trash code, probably you copied that text from MS Word, and that's why.

Here is my advice / lesson:

1. Install JCE editor Core (free)
2. Set it as default editor instead of TinyMCE in Global Configuration (Joomla Admin)
3. Open that Article again
4. Remove all content (mark and cut)
5. Switch to HTML mode to make sure that nothing left, must be blank.
6. Back to Editor (tab) and then paste Article Text using icon (Paste a Plain Text)
7. Add image again using Image icon from toolbar, and set to be on the right or left side.
8. Save and Close