take a look at the time settings. The My Cloud has to have the same time settings as your domain. Time difference must be within 5 minutes difference between NAS and DC, otherwise NAS will be placed in Quarantine. Best is to use the DC as time server - if activated. Otherwise use the same NTP and check the time zone settings on the DLs too.
Other possibility: open the computer management on your DC and check, if the My Cloud devices are flag as "trusted relationship". If not, flag them as "trusted relationship". Background: Samba server within an Active Directory domain acts as "Backup Domain Controller" to get the copy of your existing users. In some setups of Windows Server, you may need to activate the trusted relationship for the NAS to enable this copy. Normally, no client PC need a copy or is allowed to ask for the copy of the user accounts. Only NAS will do and need this copy as they have to have the domain users available in their own access rights management console.
Summary: check the time settings on the NAS and check the "trusted relationship" in computer management of your DC.