As far as I know, an XIO error means the X server dropped the connection to the xscreensaver client. So either the X server itself crashed, or it decided to disconnect xscreensaver for some unknown reason.
If the client had done something wrong, X11-protocol-wise, this would have been a more verbose "X" error, not an "XIO" error. Maybe this is the kernel OOM-killer shooting down random long-running processes, as it sometimes likes to do? "Resource temporarily unavailable" sure sounds like it could be the X11 server trying to say that it ran out of memory.
