Hey guys, I thought I've figured out the reason of can't login the soldier, and the answer is you have a space(" ") in your soldier's name!
My former soldier's name is "Zesty Is Losing This Battle!" and now it's unloginable, then I create a soldier named "Why I Can't Login" and it's not working too. (Actually it's now only display "Why") Finally I created "MySoldierIsGone" without space and it is loginable. I do find some others' situation in the forum and find something same. Also I made a poll about it and I would like to know what's your status there.