I compared the repo definitions but found nothing interesting. I even reinstalled debian-13-minimal but the error still occured.
I already set the Debug level to the max level.
I regret doing this now (because I would be very interested in why this error occured out of nowhere?) but I deleted sys-apt-cacher-ng out of frustration and set it up again with my saltstack states (so its 1:1 the same) and now it works again.
The only error that still occurs and that I get quite often is when updating debian-13-minimal, I got it even after re-creating my sys-apt-cacher-ng, but as of writing this post I couldn’t replicate it again; I will create another post when it occurs again.