Perhaps Firewall related? I think Windows asks you to identify what kind of connection you are on and if you select Public or Work it is more restricted.
Found the error! Going into the Network and Sharing Center → Adapter settings.
There I looked at the settings for the network adapter where my ethernet cable goes in, where you can enable/disable things like IPv4/6, set custom DNS and IP etc.
The first setting is called “Client for Microsoft Networks”(translated). This was not ticked.
Once ticked, I could access the OSMC as well as the attached HDD through \ \ IP and also via Network > Computers in the standard file explorer.
Hunting this error has been quite exhausting. 
And in the end the solution seems so obvious…
Thanks for helping everyone!
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Had a similar issue in accessing USB shared drives on my PC – Kodi kept asking for Username/Password under Lock Preferences. All the above did not work for me.
Solution was to create a Guest User account using Windows Credentials Manager. See link: Cannot Connect to CIFS / SMB / Samba Network Shares & Shared Folders in Windows 10 - Tech Journey
Now works flawlessly…
That’s it! I had the same problem and adding the “Client for Microsoft Networks” to my network adapter settings worked like a charm!