I've just updated my HUD from v4.13 to v5.x. I've transferred my cuffs/gags over, but the cuffs are not working. Help!

The v.5 HUD uses v.2 cuffs. The older cuffs from your previous v.4 HUD will not transfer over. You will need to setup your v.2 cuffs again, including plugins.

At the Update Switch, did you choose "HUD only" instead of "HUD + cuffs"? "HUD only" gives you an empty HUD, and is useful for when you are swapping into a new HUD because SL has borked your old HUD (so you just transfer your gear over, which are already all current). "HUD only" might also be useful if there's been an HUD update but your gear's version remains unchanged (you can tell from here or here, for example)

So, if you're updating from a v.4 HUD, you need to select "HUD + cuffs", as that will contain the new v.2 cuffs that you need to setup. Update: Now, if you verify using an old v.1 Cuff, the Switch will automatically send you a complete boxed set (i.e. HUD + Cuffs) without needing you to select an option.

To fix,

1. Go back to the Update Switch and get a new box for each gear you own (using your old "used" gear).
2. Setup the new v.2 cuffs.
3. Activate any arm/thigh plugins.
4. Finally, transfer your gags over from your old HUD.