Can't access server/shares on Win 10 machine

This topic can go down the proverbial "rabbit hole" so all I can offer are a few more suggestions and things to look at.

For a test, you can set a network share's permissions to the following:
- Shared Folder. Users - Read/Write/Execute & Others - Read/Write/Execute. (DO NOT use ACL's. ACL's are the boxes at the top. None of those boxes should be checked.)

- SMB share. Guests Allowed.

These two items (above) will open up access to everyone on the local LAN. The permissions doc (following) explains client access to a server and can be used to tighten up the share, later, if needed.

Workstation / user logon permissions for access to an OMV server. -> Permissions Doc.
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Another thing to look at, real close, is the workstation firewall. If you have a second party firewall, it can easily block local LAN traffic if it's set to high security . You might try a test with your firewall temporarily turned off.

Finally (this is a long shot) your router might be involved, particularly if one of your workstations is a laptop using a wireless connection. Some consumer routers have stricter security policies for wireless connections when compared to wired connections.
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Zitat von hauker

it's odd that one of my machine has no problem, while the other one refuses to play ball ...)

Not really. I'll go out on the limb and guess that we're talking about OEM versions of Windows, that came with PC's when they were purchased.

The first problem with OEM's, "Windows Partners", is they attempt to model the typical uses of consumer PC's. Outside of business environments, client server architectures or even peer to peer LAN's are fairly rare. The largest number of consumer use cases tend to be E-mail and Web browser Internet access so OEM's tend to tighten security for consumer PC's accordingly. (And they all do it differently.) The Windows registry (regedit.exe) or the Local Group Policy Editor (gpedit.msc) allows OEM's to "help" users by hard pinning security settings and policies in ways that are difficult to find. Then there's the add-on's that OEM's may ship with these machines, third party security app's, firewalls, etc. Finally there are security app's that users add and delete themselves. In many cases, security app's change or add registry settings and/or security policy settings that are not reversed when they're uninstalled by Windows.

Along these lines, rebuilding Windows using your existing product key might clear up some of the security barriers created by OEM's and security app's. (Make sure you have all of the hardware drivers needed before starting.) I'll acknowledge that this is not an easy route.