Power Panel does not work on a Linux host (Parallels Virtuozzo Containers for Linux, Parallels Server Bare Metal, Parallels Cloud Server). When offline management is enabled for a container, it is not accessible over ports 4643 and 8443.
A similar error is observed when one tries to restart Power Panel service (pvappd):
[warn] _default_ VirtualHost overlap on port 8443, the first has precedence (98)Address already in use: make_sock: could not bind to address [::]:8443 (98)Address already in use: make_sock: could not bind to address 0.0.0.0:4646 no listening sockets available, shutting down
Most likely, some virtual host or custom Apache module occupies the required ports (8443, 4643).
Verify that these ports are not occupied by any module or website on the node:
# grep -E -r '8443|4643' /etc/httpd/
NOTE: By default, it should show only PVA-related virtual hosts.
It is already known that mod_nss and mod_revocator may lead to the described issue.