Unable to get system shells

[Tobias Gohs]

after 2-4 days my vserver stop working. i cant connect with putty or winscp.
plesk gives me the following message after the login:

0: /usr/local/psa/admin/plib/common_func.php3:154
psaerror(string 'Unable to get system shells: exec() [<a href='function.exec'>function.exec</a>]: Unable to fork [exec /usr/local/psa/admin/bin/sysinfo 'getshell']')
1: /usr/local/psa/admin/plib/cmd_server.php3:138
get_shell_options()
2: /usr/local/psa/admin/plib/cmd_server.php3:190
get_shell_lst()
3: /usr/local/psa/admin/plib/class.DomainControlForm.php:149
DomainControlForm->assign(object of type BsDomain)
4: /usr/local/psa/admin/htdocs/domains/dom_ctrl.php3:149

Betriebssystem: openSUSE 10.3 inkl. Plesk 8.4
vserver from strato.

after a reboot through the strato webinterface it will work for another 2-4 days.

even google didnt give me the answers i need so i hope you guys can help me with this issue and i have to say that im not that good with plesk updates and shell...so be patient with me

best regards
tobias gohs