Problem instaslling applications to sites

[mjump54]

I started to add a new application for a site and when I clicked on the Add New Application button in the control panel, Plesk responded with:

Error: File exception: Unable to change directory /usr/local/psa/var/apspkgarc mode: Operation not permitted

I am guessing this is a permissions problem, but have no clue where to start. This occurs for all sites and even from the Server Panel where when I click on Application Vault, I get the same:

Error: File exception: Unable to change directory /usr/local/psa/var/apspkgarc mode: Operation not permitted