hi @nochelibre, hi @all,
@nochelibre : via email. Maybe you did not hear back because @all: the issue seems to having been solved: I just now I received a system notification about an update to be ready (linux-libc-dev): after installing it, I am now able to install linux-headers-amd64 successfully 
!
I will need to check later whether my VirtualBox / virtual machine does work again, too.
In any case: @mladen / @Purism developer: THANKS A LOT FOR MAKING THIS HAPPEN AT SUCH SHORT NOTICE 
- MUCH APPRECIATED!