I remember CIFS/SMB working fine for me, but it occasionally mysteriously didn't work on some machines even with 100% correct configuration (e.g. line in /etc/fstab). It actually happened not only on Slackware, but also on Debian, though not sure what was causing issues there.
The solution turned out to be quite dump, but it might save someone (say, me in a year) a couple hours of meaningless tries.
cifs-utils. I don't know why the errors were so cryptic that I couldn't figure out it's not installed...
sambapackage probably won't harm (although it's a server).
/sbin/cifs.mountexecutable to enable user (non-root) mounting.
The errors were really useless, I think:
cifs_mount failed w/return code = -22
cifs_mount if it wasn't installed?!)
CIFS VFS: Connecting to DFS root not implemented yet