Ok, I was having the same problems as most here. Here's how I got it fixed. All of my Arturia synths are now working. This was for the Mg Modular V, MiniMg V, ARP 2600 V, and the CS-80 V synths going from versions 1 to the new updates on the download page.
There are 2 different accounts that you need to set up. One for this forum, and one to register your synths. Contact Antoine from one of these threads and send him your name and which synths you own and the email address. He will delete your previous registration. Now RE-register with a new email address. You then can get the activation code for your synth. Then open the eLicenser program and click on the green arrow button that says "Enter Activation Code". Paste in the code and click the button on the bottom right to download the licenses from the Arturia website. NOTE: You WILL NEED internet access to do this. You should see were it says everything is ok.
I hope this helps someone get their synths working as I love the sound of them!
Now time to go play!