Check in Airport Utility > Time Capsule > Disks ‘Secure shared disks’. If is set to ‘Device password’ of ‘Disk password’, you can login with any username + the password you set for your device or disk. If it is set to ‘Accounts’, use the username/password for the account you set.
If you change it to ‘Accounts’ you should be able to use username/password for your account.