Resolution: This happens only when IPC$ share is missing in your server. Clients make remote connections with the help of IPC$ and Namedpipes slots. This is a default administrative share. This should appear in each machine on the network or your network is not going to function properly or clinets can’t connect to each other.
These shares are lost when the value of “AutoShareServer” is 0 in registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanManServer\Parameters
It should be 1.
When it happens simply restart “Server service” from services.msc snap-in. It is just to make sure that there is nothing wrong with Administrative Shares (IPC$).
Moreover, when happens again, go to a client machine and try to access server using UNC path (\\IP_Address_of_SBS).
Also you may also want to look at your DNS configuration, if you are mapping using name of the server you can try mapping few user with IP address of the server. If this keeps them connected it might be DNS issue. There are variety of other things you can check, I would start with Network cable, next go to Switch or the hub, check the network cards drivers and firmwares on the server