If things that previously worked, like a Web UI bookmark, have stopped responding, this is likely why.
RoonOS 3.0 upgrades the file sharing stack to Samba 4, which no longer allows anonymous connections. All users will need to reconnect the Data folder and provide credentials, even if it was working without a password before.
On any device running Roon, open Settings → General. Under the Roon Server section, you'll see your RoonOS device listed along with its current IP address. Make a note of it.
If you have the RoonOS Web UI saved as a bookmark in your browser, update the IP address in the URL to the one you found in Step 1. The rest of the address remains the same.
Open File Explorer and right-click the mapped drive under This PC.
Select Disconnect.
From the Computer menu (or right-click This PC), choose Map network drive.
Enter the new path: \\<new-ip-address>\Data
When prompted for credentials, enter:
Username: guest
Password: guest
In Finder, press Command+K (⌘K) to open "Connect to Server".
Enter the address: smb://<new-ip-address>/Data
Click Connect.
When prompted for credentials, select Registered User and enter:
Name: guest
Password: guest