Recent convert from a Server 2016 Essentials server to Unraid.. this wasn't an issue with the shares on that box, but for some reason is with samba? I could authenticate to a share using domain\user even though I sign into my PC as a Microsoft account and I wasn't joined to the domain.
Interestingly, if I change the permissions on the share from read/write to read-only, then back to read/write, I can authenticate and get access once, then back to the error.
EDIT: Just realized that this thread was super old.. Issue still exists on current version though,