Did you have any luck with this? I'm having the same issue with Yosemite. All the shortcuts work except those ones, I try changing them to anything and they still don't work. I even tried assigning "move right/left a space" on BetterTouchTools and that doesn't work either, even if the assignment is done by using a touchpad gesture, it seems there is no way to move between workspaces, the only way it works is using a 4 finger drag with the touchpad.
Shortcuts to move to a specific space work without issues. And the shortcuts work on the guest account. Still not working after logging out and back in.
Edit: Restarting the computer seemed to solved the issue.
Edit2: Happened again, but I think I finally discovered the reason behind this behaviour. It seems to me it's very strange bug with shortcuts.
If you go to the preferences and change the shortcut from ^+→ and ^+← to something like ^+1 and ^+2 you're going to have conflict with the "Switch to desktop 1/2" shortcut. That's not really a problem if you disable them.
Ok so you change the shortcut and you disabled the other one and now it works perfectly.
Now try doing a
killall Dock. Tachan! Shortcut is not working! Weird! Nothing you do makes it work, changing the shortcut doesn't work, nothing works!
Now change the disabled "Switch to desktop 1/2" shortcut to something like ⌘+1 and ⌘+2 and to another
killall Dock. And tada! The switching is working again!
What's happening? I honestly have no idea, it seems the conflicting shortcuts interfere if the Dock is started with the 2 conflicting shortcuts, even if one of those is disabled.
Edit3: Created a video explaining how to reproduce the defect, and how to solve it.