Attempt to update a cluster from 3.0.5 to 3.5.3-x version failed with 'Internal error' in UI and shortly after that cluster nodes started to be displayed as 'Offline' and cluster state is 'Unavailable' on main WebCP dashboard.
File /usr/libexec/vstorage-ui-backend/.rnd is owned by root user:
[root@node01 ~]# ll /usr/libexec/vstorage-ui-backend/.rnd
-rw------- 1 root root 1024 May 21 00:47 /usr/libexec/vstorage-ui-backend/.rnd