I had the exact same issue as the OP in my Windows domain environment. This was extremely frustrating since the SMB shares were working fine for almost a month, then suddenly I could not connect to them with the very account I used to set the permissions with! I had used a domain admin account to set the permissions on the shares and granted full access to "domain admins", and less access to a couple user groups as appropriate.
I discovered (from running the Fix Common Problems full scan) that UnRAID saw my domain admin account as a "domain user". I looked in Active Directory and realized it was a member of both groups. I set Domain Admins as the primary group for this account, removed domain users, and immediately I could access the UnRAID shares with this account again.