@kerouak
This is likely a network control gone awry, i.e. the problem may not be on the endpoint.
Some things to check:
• Check the logs on the NAS. Compare the log entries for a Mac that succeeds with the iMac that's failing. If anything, this will be the best source of clues as to what's really going on.
• Check the controls on the NAS. Does it limit access by building, department, network segment?
• Unplug the network cable from the iMac and plug the same cable to another Mac (e.g. a laptop), configured the same. Does the second Mac, when connected via the iMac's network cable, also fail to connect to the NAS?
• Conversely, if possible, try physically moving the iMac closer to another Mac that is working — and swap their network cables. Does the iMac still fail to connect to the NAS?
• Check Active Directory for conflicting, obsolete, or duplicate computer objects for this host. (You're a domain admin, so you've likely done this already - worth a second look, though.)
• Check with your security group and networking group. Are their network controls (firewalls, routers, NAC, etc) blocking this Mac from accessing the NAS?
• Check your iMac endpoint for any security or network agents that are required by your network but are not installed or failing on the iMac
Etc — you get the idea. If you can eliminate the endpoint as a source of the problem, then the problem is likely to lay elsewhere in the network.
Good luck! Please report back and let us know what the problem was.
