Note that I have also cross posted this to Synology forums. I'm posting here because I'm not sure if the problem is with Windows or with the Synology server:
Background
Dozens of workstations with several Synology servers.
All workstations running Windows 10 Pro.
For the past year, and up until about a week ago, all workstations were able to access all Synology servers fine.
Problem and Symptoms
Now just starting about one week ago, ONE workstation is unable to access ONE Synology server via Windows Explorer.
It doesn't work with \\UNCname
nor with \\ip.ad.re.ss
However, I can ping the server just fine using the ip.ad.re.ss and nslookup for UNCname returns the correct address.
Conundrum
If this were happening to all my workstations, I'd assume the Synology server was at fault. But all the other workstations have no problem accessing this one Synology box. If this were happening with all Synology servers on this one machine, I'd assume the workstation was at fault. But the workstation has no problem accessing the other Synology boxes on the same network, and up until a week ago had no problem with the specific box in question.
Again, it is just the ONE workstation suddenly having problems with ONE Synology server.
Nothing has changed on my end, except of course that there are automatic updates for both Synology and for Windows 10… so again I have no idea where the problem lies.
Best Answer
There are a number of things you can check... that might give you more insights into what is going on.
Ping can be a useful tool, but can also give you many false-positives.
... the list continues.