We have the same error with Reset password.
After investigating, I see that the Password hash algorithm of WP 4.3 has changed. It means the Check_password() logic also changed. But in Theme my login, the old algorithm causes this problem.
※New changes WP 4.3
$hashed = time() . ‘:’ . $wp_hasher->HashPassword( $key );
※Theme my login 6.3.12
$hashed = $wp_hasher->HashPassword( $key );
We have to wait for Theme my login new version, I think that.
Does anyone have any other ideas?