I was experiencing this issue on a Windows 2019 Server VM. The local Computer policy (Computer Config > Administrative Templates > Network > Lanman Workstation > "Enable insecure guest logons") was set to "Not Configured". The description suggests this means that access to such shares is allowed.
I then took a look at the following post:
which indicates that there is a registry key that should be set for this setting (i expected the policy to set it to 1) however the actual registry key is set to 0 by default on the install i had. Setting it to 1 immediately allowed my WS2019 machine to connect to the unRAID Host's SMB shares.
The registry key: HKLM\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters\AllowInsecureGuestAuth key, value set to 1 worked for me.
