Hey man, I'm pretty sure I can help you and anyone else who has this problem. But you have to be very careful doing this and it's at completely your own risk as it requires a delicate hand.
I fixed my cuboid by doing this so if you do it right you should be in business.
The 510 connector on the cuboid has most likely been pushed down too far by your RDA/Tank. You'll notice that the rubber insulator around the 510 connector is probably protruding too far up the sides of the 510 connector. The following steps will lead to minor damage of that rubber insulator (tiny o-ring).
Take the smallest flat head screwdriver you can find (the small types used for PC and cellphone repairs), and very carefully slide it into the side so it's jammed between the 510 connector and the insulator. Then use that leverage to slowly pull the 510 connector towards you and away from the o-ring. You want to do this slowly and carefully on all sides of the 510 connector so that it begins to slowly come away from the o-ring but making sure you don't break it or pull it completely out of the cuboid.
The idea is to get that 510 connector about 0.5mm away from the o-ring because it's the o-ring causing the short between the 510 connector on the Mod and the 510 pin on your tank. You want metal to metal contact without the o-ring touching your atomiser.
You will undoubtedly start to cause small cuts and dents into that o-ring but you want to keep that damage to a total minimum. Also be careful not to chip away any metal from the 510 connector as this also happened to me slightly. It might take a few tries before your shorting issue goes away but it worked for me.
Here's a pic of what mine looked like after this fix.
EDIT:
If you can prove that you're still within warranty of the product then my advice would be to send the product back to the company that sold it to you or straight back to Joyetech (who replaced the unit completely). The way I found out that this was the issue was from a user that received a response directly from Joyetech. It's a great Mod and worked for months perfectly for me until this issue occurred. Unfortunately it's an issue with design/build quality and up until now has been the only let down for me. Still my favourite mod though and I would happily buy a new one if I had to, just being more careful with how tight I screw down my tanks.
Good Luck!