.xml file not generated in /custom folder
Checked it again, the missing .xml file is not generated.
De-installed HTML editor with the extension manager.
Made over the page with the editor.
Once again, the .xml file is not generated.
So the 404 error is quite logic.
The text area is dynamically created and named by ADDT server behaviour Forms > Create Dynamic Form wizzard.
This all is pretty annoying, especially because I cannot re-install the previous version (that did not work properly either by itself).