Over the past year, I've had to re-install Simplify 3D SEVEN times.
I know the number because I have a little "(7)" at the end of the filename from when I download each time, fresh.
This is after a clean install, reboot of the system, proper authentication (signing in), and activating.
This computer always has Internet. In fact, I was researching the problem each time, in these forums, so clearly Internet is working.
The fact that it works after an uninstall/reinstall leads me to believe this is an activation bug/problem.
I had this same problem with previous versions, so I feel like I have to reinstall this software every month or so.