Don't log in. Instead, click on Discover (on the top of the screen of WDW). Then click on either the accommodation link or theme park link (depending upon where the first restaurant you want is located). Then click on 'book reservation." THEN enter the requested information, and it should go right through.
Many have had the same problem--can't logon from the main page or from the 'my vacation' page, but can doing it this way. GOod luck!