On a container with a Unix domain socket, owned by a user other than root, the socket becomes owned by root after online migration from one node to another.
[root@vz ~]# ls -l /var/run/apache2/wsgi.*.sock srwx------ 1 www-data root 0 Feb 14 12:11 /var/run/apache2/wsgi.2416.0.1.sock
[root@vz ~]# ls -l /var/run/apache2/wsgi.*.sock srwx------ 1 root root 0 Feb 14 12:22 /var/run/apache2/wsgi.2416.0.1.sock
Apache processes will no longer be able to open the socket.
The issue is recognized as a kernel bug with internal ID PSBM-25014.
Fixed in CU-2.6.32-042stab085.17.
To resolve the issue immediately, it is required to restart the corresponding failed service in order to bring its functionality back after the migration.