Don't go to the MY VACATION part. Instead, go directly to the red part below (it's not red on Disney's site) on the dining reservation page of the restaurant that you want & put your info in there: (From the Disney website)
To make your reservation, please enter the requested information:
Update Your Profile
Are you an Annual Passholder and/or Florida Resident? If so, please add this information to your account profile so we can tailor your dining search.
Retrieve Your Resort Reservation
Do you have a Resort reservation for the same days for which you're making dining reservations?
If so, please retrieve your reservation.
This is
EXACTLY what I did to get it to work. It let me make the first reservation for 12/12, but that was it and I almost passed out.

Then I saw the bit about "retrieving your resort reservation" and did that......Once I did, the calendar opened up all 10 days. Whew!
BTW, the only way I got the 404 error message to go away was to log off and get back on. That worked for me.
But here's
my horror story. Once I got my 10 day window, I was cruising along and thought I had "booked" all 10 spots, since it let me select them. Then I confirmed and even gave my CC#. At that point, I got a message saying that some of my ressies couldn't be confirmed after all. ACK!

It only gave me 5 out of 10. So I thanked my lucky stars for those and started over for the others. I tried to book the next 3 that I wanted most and only got 2 of them, even though the screen initially indicated I could have all three.

And so on and so on. At the end, I just kept plugging in Flying Fish for 7:00 over and over and over. Every time, it would say the time was available, but the very last screen would say the equivalent of "too bad, so sad." Finally, after about 7 tries, I got it.
I made certain to get all my confirmation numbers and that's when things got really weird. I had
TWO Bomas and
TWO Biergartens, each on the same night.

Apparently, one of those times when it gave me the "you're out of luck and you need to try again" message, the ressie had gone through anyway. So when I tried again and got it, (for what I thought was the first time) I was actually double booking.
But I'll wait until tomorrow and phone dining reservations to make double dog certain I have one in hand before I cancel the other. Too bizarre. Long story short, it took me from 2:00 Central to 3:30 Central. All because I had to put them in my cart 1-2 at a time, check out and start over. But I can't complain too much since I got what I wanted and was able to start earlier than expected!