It’s a puzzle - entering \\<Nucleus+ IP Address>\ into Windows Explorer should work. Have you tried entering \\nucleusplus\ ? Does this also fail?
You should not need to enable SMB v1 in Windows - Roon OS supports SMB v2. I wonder if Windows has disabled guest access (possibly during a recent Windows Update??)? See:
BTW, you wrote:
I hope that you are not ripping the CDs directly to Nucleus storage without first stopping the Roon Server of the Nucleus+? Ripping directly whilst the Roon Server is running runs the risk of Roon failing to identify albums properly.