FATAL:zygote_host_impl_linux.cc(124)] No usable sandbox! Update your kernel or see https://chromium.googlesource.com/chromium/src/+/master/docs/linux_suid_sandbox_development.md for more information on developing with the SUID sandbox.
I googled and found this solution at superuser-questions:
On Debian userns is compiled-in but disabled by default. There's a Debian-specific patch to the kernel that adds sysctl knob: kernel.unprivileged_userns_clone. You have to add a new entry in /etc/sysctl.d and set it to 1
You can do it manually by typing:
# echo 1 > /proc/sys/kernel/unprivileged_userns_clone
This is a temporary solution and only works until reboot.
For a permanent solution:
# echo 'kernel.unprivileged_userns_clone=1' > /etc/sysctl.d/00-local-userns.conf
# service procps restart
After issuing the above command and launching Iridium, it runs flawlessly.