Insert a background in Templet Error: undefined

I like to insert a png as a background in helix ultimate -> layout -> Main Body -> Menue -> Style -> select media.

I can see the picture and i can press [Applay] - Button.
The Message is
Layout Settings successfully
Changes applied successfully

If I press the Save Button upper right the Message is

                                                            Error: undefined

                                                            and a rotating circle with Drafting...

I have
chanche the browser
Clear the Cach in Joomla System
using a small picture

Who can help me?

Thanks Aimo Lars Piegholdt