Unable to get system shells: Empty error message from utility.

[GnoStiC]

we have plesk 7.5.4 and we get;

0: /usr/local/psa/admin/plib/cmd_server.php3:135 psaerror(string "Unable to get system shells: Empty error message from utility.")
1: /usr/local/psa/admin/plib/cmd_server.php3:185 get_shell_options()
2: /usr/local/psa/admin/plib/class.DomainControlForm.php:170 get_shell_lst()
3: /usr/local/psa/admin/htdocs/domains/dom_ctrl.php3:154 DomainControlForm->assign(object of type BsDomain)

errors. we checked the logs and found that /usr/local/psa/admin/sbin/wrapper is null.
---s--x--- 1 root psaadm 0 Jul 3 12:25 wrapper

we have no idea how did this happen, and would like to know why?
any suggestions and resolutions?