Wie kann ich das beheben?

This is result of plant growing those portions of stem in lower light conditions. The top part won’t fall off but you also won’t fix the way it looks as plant can’t go back and ‘redo’ portion it already grew. Maybe if stem thickens those parts will thicken a bit too but they won’t ever grow even with rest of the stem. You can only take care to avoid it going forward.

To prevent weaker growth and etiolation in inadequate amount of light in winter, many growers in colder regions choose to push many of succulents and cacti to stop all growth with lower temperatures and reducing water (also because some plants need dormancy for long term health and to initiate flowering. And to make it more complicated some succulents are winter growers needing high light in winter and dormancy in summer) – even when it makes plant look worse short term, your euphorbia for example would probably loose leaves. 

Supplementing artificial light may also help.