The issue was because the customized filesystem.squashfs was missing the /proc and /tmp directories.
Ubiquity expects these directories to be present, because they are not automatically created during the installation process.
Including empty /proc and /tmp (and /run) directories solved the problem.
The issue was because the customized filesystem.squashfs was missing the /proc and /tmp directories.
Ubiquity expects these directories to be present, because they are not automatically created during the installation process.
Including empty /proc and /tmp (and /run) directories solved the problem.