impossible to access network share from system context

For a number of reasons I need the system context to be able to access and read certain public network shares.
These shares are located on a samba server, and are public read to everyone. Environment is WORKGROUP.
My Win10 machine can ping the server sharing the folder, and can properly read the folder content unless the action runs under the user nt authority\system (the one I need to run the action).
The machine has already been patched with the change
[HKLM\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters] AllowInsecureGuestAuth REG_DWORD 1
but this seems to have no effect on this specific issue.
The machine is a fresh install, and anyway this behaviour is happening systematically
(and since a few weeks). For running processes under system context I use
PSexec.exe -i -s cmd
by Mark Russinovich
In Win7 the behaviour is as expected (the result of the red cmd is the same than the yellow cmd).
In my specific case it does not make much sense to create a user on the samba server, because shares are absolutely public.
Is anybody having a similar issue??