This is not a problem caused by the fCraft server nor software.
There is a workarround for this on http://www.worldofminecraft.com you might wanna follow.
source: http://www.worldofminecraft.com/content ... ng-accountLunick wrote:So I've had some people complaining that they can't login to the client because they migrated their account to a Mojang account.
When you login to the client, use your email instead of your username.
When you choose a server, look at the url at the bottom and remove your email and put in your username.
Simple as that.