Cannot access router HDD via SMB from one Windows machine while it is working from another

I have an HDD attached to a router (O₂ SmartBox – a custom router from my provider). The router exposes the HDD using SMB/samba.

While I can connect to the HDD from one Windows 10 2004 Pro PC ("PC1"), it does not work from another Windows 10 2004 Pro PC connected to the same network ("PC2").

The PC1 does not list the router in the "Computer" list in Network view in File Explorer. When typing the disk UNC path manually (\\router\share), I'm prompted for credentials (which there are none). I've tried to submit empty credentials or some random values. Neither works. Eventually I get 0x80004005 (Unspecified error). I can ping the router (obviously).

The PC2 sees the router in the Network view and I can access the shared HDD without any credentials. The PCs see each other and can access each other's shared drives.

Both PCs see the router in "Media Devices" list Network view in File Explorer. But when using "Open Media Player" command (what is not what I want to use anyway), PC1 does not show the router HDD in the opened Windows Media Player, while PC2 does.

The router also offers an FTP interface. That works on both PCs.

Both PCs are connected to the router with WiFi. Both PCs have the network set as private. In the "Wireless LAN adapter Wi-Fi" section of ipconfig /all I do not see any significant difference between the PCs.

The PC1 is an old one, so I might have tweaked something in its settings in the past. The PC2 is a fresh one, so most its settings are defaults.

I've tried to run "Shared Folder" troubleshooter from Windows Settings. It reports "No issues detected" for the \\router\share.

Any idea, what should I do to resolve or debug the problem? I can imagine, I'll have to provide more information, but I do not know what.

I've found I need to enable SMBv1 and that the router probably does not support a newer version. Its strange SMBv1 was enabled on the new PC, while disabled on the old, as I would expect the opposite, even contradicting Microsoft documentation, which states SMBv1 is not installed on Windows 10 v1709 by default.

Anyway, I've followed Microsoft's instructions for installing SMBv1, which resolved my problem; obviously, it's not anything to be recommended in general:

• How to Detect, Enable and Disable SMBv1, SMBv2, & SMBv3