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

Must 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)