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Perfect! Then it is because of the old packages. Unfortunately I cannot update packages. But everytime the error occurs, I can use TL2020 via the terminal. This is enough for my purposes. Thank you very much for trying out!

Hi Ijon, hi Stefan,

thank you very much for your help! I had installed texlive via
sudo apt-get install texlive-full
and it turned out to be version 2019.
Now, I installed TL2020 and I could compile the file using the terminal. This works and produces the expected output. So the problem is solved ...

Hello everyone!
It seems to me that the algorithm for floating environments (like figures or tables) sometimes entirely fails to place the environment anywhere. Here a minimal working example:
\documentclass[11pt, a4paper, UKenglish]{scrreprt}
\usepackage[UKenglish]{babel}
\usepackage{tikz ...