Hello @Blaise ,
I think that your problem may be related to other reported problems in this forum and we may be getting closer to an understanding of the root cause. The problem seems to affect older Network Attached Storage devices and Windows 2003 and earlier. I have included part of another post of mine here, adapted to your scenario.
The authoritative source of information about SMB is the Microsoft specification "[MS-SMB2]: Server Message Block (SMB) Protocol Versions 2 and 3".
Section "3.2.4.2.2 Negotiating the Protocol" of this document says:
When a new connection is established, the client MUST negotiate capabilities with the server. The
client MAY<111> use either of two possible methods for negotiation.
The first is a multi-protocol negotiation that involves sending an SMB message to negotiate the use of
SMB2. If the server does not implement the SMB 2 Protocol, this method allows the negotiation to fall
back to older SMB dialects, as specified in [MS-SMB].
The second method is to send an SMB2-Only negotiate message. This method will result in successful
negotiation only for servers that implement the SMB 2 Protocol.
The footnote <111> says:
The Windows-based client will initiate a multi-protocol negotiation unless it
has previously negotiated with this server and the negotiated server's DialectRevision is equal to
0x0202, 0x0210, 0x0300, 0x0302, or 0x0311. In the latter case, it will initiate an SMB2-Only
negotiate.
What might be happening is that your clients are attempting to reconnect to the Windows 2003 PDC (after an interruption in the TCP/IP connection) using an SMB2-Only negotiate message. Windows 2003 seems to just silently ignore this negotiate message, perhaps expecting the first SMB message on a new connection to always be a multi-protocol negotiation.
If you issue a "net use" command on a client that can't connect, you might see something like:
New connections will not be remembered.
Status Local Remote Network
-------------------------------------------------------------------------------
Disconnected \\ray\c$ Microsoft Windows Network
The command completed successfully.
If you do see that, try issuing the command "net use \ray\c$ /d" for all of the connections to that server (e.g. "ray"). The reason for doing this is that the client remembers that it negotiated an SMB2/SMB3 connection with the server and skips the multi-protocol negotiation; removing all connections to the server also causes this "remembered" information about protocol capabilities of the server to be deleted.
After this, it should be possible to re-establish connections to the Windows 2003 system.
Please let us know what experiences you have with this approach - it would help to build a body of evidence for the problem.
Gary