Thanks for your answer.
I didn't know the concept of httpredir.debian.org. That's pretty smart, I think.
And also I was not aware of /etc/apt/sources.list.d/armbian.list.
The errors I got in using apt-get update where produced by apt-cacher-ng, which I'm running on another machine, combined with the httpredir.debian.org URLs. Without apt-cacher-ng everything was fine. Also using debian.tu-bs.de as mirror with apt-cacher runs without errors.
I'm wondering why...
But everything should be fine now, thanks for the good work! I came along this project because I searched an image for my C1 which is able to run fail2ban, and it was the first one that did the trick :-)
Frank