Sure, I'm just trying to find out whether the problem is with the expectations of zef and I expect the developers to work under a non-root user, which is why I'm trying to get you to replicate their situation more closely.

What is your working directory? The error message suggests that it is /tmp, and maybe /tmp has special permissions or a limited size. I suggested to install it below $HOME in a separate directory, but you know your system better and can maybe suggest a different directory that is not special.