Plesk node has status "Available" and "Unregistered" in OBAS. Plesk Panel itself works correctly: port 8443 is available from OBAS node. If I run %plesk_bin%\sysinfo.exe all inside the Plesk container, I get the proper output.
Rebooting of Plesk node resolves the issue temporary.
Plesk 9.x is installed in a container of Virtuozzo Containers 4.0. These versions are outdated and reached EOL:
The best solution is upgrading outdated Plesk and Virtuozzo to the latest versions. Refer to these KB articles for more detail:
Important: upgrading Plesk Panel 8.x to version 10 or 11 is not officially supported. If you need to do that, please contact Plesk Technical Support for assistance.