Why is my machine unable to mount my SMB drives ("CIFS VFS: Error connecting to socket. Aborting operation", return code -115)?

I found that switching off IPv6 solved a very similar problem for me. My Ubuntu 12.04 box is hooked up to an enterprise network dominated by Windows users and I don't have any authority over the server. Following the tutorial at http://www.noobslab.com/2012/05/disable-ipv6-if-your-internet-is.html solved my problem. (Save a click: Edit /etc/sysctl.conf to insert three lines of the form "net.ipv6.conf.*.disable_ipv6 = 1", replacing * with "all", "lo", and "default"; then give the shell command "sudo sysctl -p".)

My internal IP ended up in my NAS's blocklist. I removed it; problem solved. Used smbclient to troubleshoot.

EDIT: I am much more familiar with GNU/Linux these days, and if I were attempting to debug this today, I would check out hosts.deny and iptables, one of which (or both) is likely used to implement the blocklist/blacklist in my now decommissioned commercial NAS.