Please, read the documentation carrefully: i-MSCP inside a LXC container (Managed by Proxmox 4.x)
The permissions (through apparmor profile) must be given on the host.
We start now LXC Proxmox 5 with I-MSCP 1.5.3 on Debian 9.3 and with the documentation all installed perfectly and works.
Thanks very much for reply