Hi Garry,
thanks a lot for your detailed reply!
I don't know yet, which item caused this problem - perhaps it was an imported and ungrouped SVG. But I could solve the problem for now. I will describe it, perhaps it may help someone else.
Some time Scribus must have added this blank line style in one of my projects, perhaps during editing an imported SVG...
Responsible for this style is the following tag in the SLA file:
<MultiLine Name=""/>
When I insert an object (eg a Textframe) into the scrapbook from a document with this blank line style, Scribus also copied the Tag <MultiLine Name=""/> into the corresponding SCE file. Now when I insert this object from the scrapbook into a new document, scribus importes the blank line style as a style of the object in this file, and so on. Clear? I know it seems crazy, but it's true!

My solution:
I searched all my SLA and SCE files and removed the Tag above, wherever I found it, manually.
From this time on the "ghost style" has disappeared! Happy Halloween!?

Regards
CiTa