I've been having this problem for 5 days and counting now. Or maybe 6 at this point. The latest I heard was that my user name is a http://verizon.com account and I need a http://verizon.netaccount and that a specialist has to fix that. I have not heard back yet and it is still not working for me.
Other suggestion given by tech support was:
Please try http://vz.to/XJyaEG to sign the Terms of Service agreement. Maintenance was done this week which reset the TOS for you.
Of course that didn't work for me no matter how many times I tried, because I do not have the proper set up to log in with a verizon.net account which is required for FiOS now but was never set up for me. Hopefully you have better luck and it's a quick fix for you.