As Yandex is not providing official Yandex API for "music", we have to deal with a workaround, which don't work for all users.
Unfortunately we don't know why some Yandex Music accounts can't be logged to Soundiiz right now; still trying to figured out.
However, if you can't login Yandex Music to Soundiiz, be sure to disabled the 2FA (Two Factor Authentication) before trying to connect Yandex Music to Soundiiz. If still not working after that, it means that we are not able to login to your account.
Thank you for your understanding.