I've been chasing down this issue since New Years day, and I finally have conclusive proof that the problem is caused by Bitdefender Firewall (so what else is new?).
The situation backstory involves the presence of two media players (Oppo BDP-103 and Oppo UDP-203), as well as my LG OLED C9 "smart TV", all of which have the ability to "browse" the drives/folders/files of SMBv1 servers (located on PC's) on the LAN. My LAN includes two Win7 desktop PCs and three Win10 laptops.
To facilitate media on the PC's being played (to TV and sound system) through the Oppo players, all five of my PC's have had their "advanced sharing" set to "enable streaming media" which facilitates their acting as a "media server". The Oppo players understand this technique and act as a "media client", through which all content located in the folders on the PC which are included in the PHOTO, MUSIC, and VIDEO Libraries are accessible via the Oppo players. Files which are not located in folders included in these three Library definitions are not accessible from the Oppo using this technique.
But the Oppo players also have the ability to act as if they were a generalized "Windows Explorer" application, able to browse and retrieve (for viewing or playing to TV and sound system) ALL of the shared drives/folders/files on any PC which is running an SMBv1 server application (which is the "partner" to this "file browser client " running on the Oppo players. Win7 has support for SMBv1 built into it, but it's not enabled by default. However it's a simple task to enable SMBv1 file sharing. Win10 also support for SMBv2 and higher built into it, but it is also a simple task to "add the optional feature" of SMBv1 file sharing into Win10. And I have done just this, enabling SMBv1 support on both my two Win7 desktops as well as my three Win10 laptops. This therefore provides access to ALL of the drives/folders/files of all five PCs for viewing/playing media files using the Oppo players, no matter whether this data is in a folder that is defined as part of a Library or not.
That's the background. Now for the problem description: when BD Firewall is active, the three SMBv1 servers running on my three Win10 laptops ARE NOT VISIBLE TO THE OPPO PLAYERS, so that none of the shared media data on these Win10 laptops is accessible to the Oppo playerst!! But all I have to do temporarily (or permanently!!) turn off Firewall in Protection, and bingo! the three SMBv1 servers now become visible and full browser functionality by the Oppo players operates normally.
This doesn't seem to be an issue with SMBv1 as implemented in Win7. The SMBv1 server running on my Win7 desktops seem to be accessible from the Oppo players no matter whether BD Firewall is enabled or disabled. This issue only seems to affect SMBv1 as implemented in Win10.
Here is the proof. First, a pair of images showing the "Network" as seen by the Oppo player, one with BD Firewall enabled on one of my laptops named T495T2 and the second with BD Firewall disabled on the same laptop. When BD Firewall is enabled on T495T2, the SMB server object for T495T2 DOES NOT APPEAR (i.e. there are only 14 network objects shown). When BD Firewall is disabled on T495T2, the SMB server object for T495T2 DOES APPEAR and there are now 15 network objects shown.
The second pair of images is the output of a neat utility named "Lan Scanner" (i.e. Lanscan.exe) which queries, sorts, and then enumerates all VISIBLE network objects identified by their Windows "workgroup" name, also showing their computer name, IP address, and MAC address. The method of query us through the repeated use of the NETBIOS command: NBTSTAT -a <computer-name>, where each <computer-name> is from the list returned through the NET VIEW command. This is all done internally by Lanscan.exe with the output presented in a very nice tabular format.
Note from this pair of images that once again, when BD Firewall is enabled on T495T2 the output of Lanscan.exe (run from any other machine on the LAN, other than T495T2) fails to show the expected information for T495T2 (because the PING to T495T2 which is part of NBTSTAT fails, caused by BD Firewall on T495T2). But when BD Firewall is disabled on T495T2, now the PING is successful and the information requested by Lanscan.exe is returned, processed, and presented properly.
Note from the Oppo network images that the "media server" present on all five PCs is apparently not impacted by BD Firewall. Regardless of whether BD Firewall is enabled or disabled (on both the two Win7 and three Win10 machines) each PC's "media server" enabled through the "streaming video" option of "advanced sharing" is still visible to the Oppo players. It's only the SMBv1 servers running on the three Win10 laptops whose visiblity seems to be impacted by BD Firewall. And again, the Oppo players only support SMBv1, not SMBv2, and that is why this BD defect is so impactful.



