Comment 1 for bug 156444

In fact this happen because I didn't fill all field when I created the account.

To fix this :
$ rm -rf ~/.gconf/apps/telepathy/mc/accounts/salut*
$ killall gconfd-2

Create an account without any empty field. Close the account manager window. Then this account will still be displayed when reopen the window.

You also can delete mc account using gconf editor and removing all keys of the account to remove.