@Dominik: do you remember if you ticked the "download updates while installing"?
I suspect that if you do select "download updates while installing" and get unlucky, 'update-apt-cache' will race with copy_all():
1) update-apt-cache created /target/var/cache/apt/archives/.
2) copy_all() (re)moved /target/var/.
3) update-apt-cache created /target/var/cache/apt/archives/partial/.
4) copy_all() attempted to create /target/var, but it existed again due to (3).
@Dominik: do you remember if you ticked the "download updates while installing"?
I suspect that if you do select "download updates while installing" and get unlucky, 'update-apt-cache' will race with copy_all():
1) update-apt-cache created /target/ var/cache/ apt/archives/ . var/cache/ apt/archives/ partial/ .
2) copy_all() (re)moved /target/var/.
3) update-apt-cache created /target/
4) copy_all() attempted to create /target/var, but it existed again due to (3).