Earlier, we we're unable to access the Plesk Control Panel for the admin and clients. It doesn't show the login page or the usual error Page cannot be displayed.
Offline management was disabled.

From Parallels KB,

Virtuozzo Power Panel has the following login mechanisms:
OFFLINE_MANAGEMENT parameter is set to "yes", VZPP accepts the VE "root" login
with the corresponding password when connecting to VE IP address.
OFFLINE_MANAGEMENT is enabled and Plesk Control Panel is installed in the VE -
loging in the VE using Plesk credentials (user admin with the corresponding
password) is supported on the VE IP address. In this case, however, the
management capabilities available via web interface are limited.

The Service VE administrative credentials (user vzagent0 with the
corresponding password) are supported as well. Upon administrative login all the
VEes on the Node are manageable.When offline management is on for a VE and the
VE has Plesk installed, Plesk panel interface works via Service VE (in other
words, port 8443 is forwarded to Service VE). In this case some additional
Virtuozzo-related functionality is added to Plesk interface.


Enable offline management.

1. Go to the hardware node using your favorite ssh client. E.g. putty.exe

2. Type:
vzctl set (node id) --offline_management yes --save
Example. If the node ID is 101 then type:
vzctl set 101 --offline_management yes --save

That's it. You should now be able to access the Plesk control panel.

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