One of the updates that came down the wire to the machine I'm running on the insider slow ring in the last few weeks had the effect that trying to access the SMB share on my router gave a message like
\\Remote-Server\Path is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions. etc.
Breaking out Wireshark and comparing a machine still on build 1607 (local account login) with the test machine (MSFT login) showed that the SMB handshake would perform the initial exchange of
Session Setup AndX Request, NTLMSSP_NEGOTIATE and
Session Setup AndX Response, NTLMSSP_CHALLENGE, Error: STATUS_MORE_PROCESSING_REQUIRED, but the test machine would not then emit a
Session Setup AndX Request, NTLMSSP_AUTH, User: Machine\User packet. Further experiment would be needed to tell whether this is because I'm running that machine with a MSFT login rather than a local account, rather than it being an insider build, but in the end, the fix turned out to be going to
Control Panel\User Accounts\Credential Manager and creating a new Windows credential just for the
Remote-Server address, with username and password sufficient to log on to the share (so for a wide-open read-only share, any username and an empty password will do).