Question : specified network name is no longer available

Office A - windows xp pro as the file server connected to the internet through a dlink vpn router (di-804v) and a cable modem.
 
Office B - windows xp pro connected through a dlink vpn router (di-804hv) and a ADSL modem.

I can copy a file (sample.db 4009k) from A to B but, I can't copy the same file from B to A without getting the error "cannot copy filename - specified network name is no longer available"

Other symptoms - Data file sits on the server at 'Office A' and is accessed by accounting software on local workstations (runs fine!). The trouble begins when the workstation at 'Office B' tries to run the software accessing the data file over the vpn. I receive the error message  "cannot lock network file. operating system network error".

Do both these problems have the same root?

Answer : specified network name is no longer available

You indicate 4 machines, but only list 3 in LMHOSTS.  They should all be in LMHOSTS.

Besides that, the NG info you posted re: packet fragmentation over Cisco DES VPN may be of consequence.

I assume that your default route is set to the local VPN IP on each site.

To disable oplocks:

On dedicated servers running Windows 2000
Professional, Server, Advanced Server, or Windows
XP Professional, under this key in the registry:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters\

Create the following:

Value Name: EnableOplocks
Data Type: REG_DWORD
Parameter Value: 0 Hex (False)
(The default value is 1 (True))
(If your server is not a dedicated server you will also
need to create the workstation keys listed below.)

On workstations with Windows 2000 Professional, or
Windows XP Professional, in this key:

HKEY_LOCAL_MACHINE\System\
CurrentControlSet\Services\LanmanWorkstation\
Parameters\

Create the following:

Value Name: UseLockReadUnlock
Data Type: REG_DWORD
Parameter Value: 0 Hex (Disabled)
(The default value is 1, Enabled)

Create the following:

Value Name: UseOpportunisticLocking
Data Type: REG_DWORD
Parameter Value: 0 Hex (Disabled)
(The default value is 1, Enabled)

Create the following:
Value Name: UtilizeNtCaching
Data Type: REG_DWORD
Parameter Value: 0 Hex (Disabled)
(The default value is 1, Enabled)


If you aren't sure on this or have any misgivings about editing the registry, then do NOT do this.  If you do try this, back up the registry first.