In order, to use the reiserfs file system with i-MSCP, you must follow these steps:
Edit your /etc/fstab file to add the attrs option for your device (eg. device containing the /var partition). For instance:
UUID=74699091-3ab8-43f2-bdd5-d1d898ab50fd / reiserfs notail 0 1
should be updated to:
UUID=74699091-3ab8-43f2-bdd5-d1d898ab50fd / reiserfs notail,attrs 0 1
Once you did this, you can remount your device. For instance:
# mount -o remount /dev/disk/by-uuid/74699091-3ab8-43f2-bdd5-d1d898ab50fd
And then, normally you must be able to use i-MSCP without any trouble (a least with the immutable bit). If needed, you can find the uuid of your device, with the following command:
# blkid device
where device must be replaced by your device path (such as /dev/sda1)