[crashnburn]

Thank you for your solution. I came to the same conclusion, that my earlier problem of the so called "invalid argument" had to do with SMB. But I only saw it on the RPi LibreElect version. This version of Kodi implements SMB using all three newer versions. However my version of Windows 10 only provides for SMBv1. So if I wanted to use SMB, I had to set Kodi to SMBv1 max. This was the only way I could connect to shares on my Local Area Network.
Adventures in PC Land should be the title of a best seller.

Thank you for your solution.
I've been banging my head against this proverbial wall for some time now. Up until very recently I was using kodi on windows 10 PC's and quite comfortable too. I bought an Amazon firestick 4k and the problems came. adding a video source via windows network (SMB) was not working for me on kodi 18.5. i tried a few methods and steps including making changes in the router settings, the local machine network adapter settings and SMB features.
in the end the last step was to set the "SMB client" in kodi to max vers. as SMBv1.

now i am getting the user credentials prompt and can access network shares. 
Thank you very much. 
if more details on the steps i took are needed. i can do so in a later post if needed.   

Please do share details. What version of Kodi, installed from where & how, what steps and then what setup on Network and Windows & SMB Share.

Similar issues on FireStick 4K 
18.x would show nothing. 
Got 17.6 - worked fine with Shared on one Ext HDD. 
Now another Ext HDD attached to same PC with similar security/ permissions - Can see Share, cant enter it - INVALID ARGUMENT. 

Where is this SMBv1 setting. I'd like to try it.

Update:

Found it. Installed latest Kodi 32 bit on Firestick 4K -
But cant figure out why from same PC, same powered USB HUB, all shares SEEN via SMB v1 (so IP is not an issue or reason),
but one HDD shares are accessible, while others give INVALID ARGUMENT.

If not, I wonder whats with this weird Share behavior bias on 2 diff HDD shares.