Cannot connect to local network computers or RU Server on macOS

Symptoms

On a Mac, Viewer cannot connect to computers in the local network by IP address or computer name and cannot sign in to an RU Server located in the same network. The same can happen to Host or Agent running on a Mac when they connect to an RU Server in the local network. Typical signs:

• Connections over the Internet work, including Internet-ID connections.
• Ping and other programs on the same Mac reach the same computers. Viewers on Windows or Linux connect to the same Hosts without problems.
• The Host on the other end does not log any connection attempt.
• The problem appears right after Remote Utilities was updated or reinstalled on the Mac.

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Cause

macOS asks every application for permission to use the local network and keeps the answer in System Settings→Privacy & Security→Local Network. The permission is tied to the exact application files. After an update the stored permission no longer matches the new files, and on some macOS versions the system silently blocks the application's local connections instead of asking again, even though the switch in System Settings still shows the application as allowed.

This is a macOS behavior, not a Remote Utilities setting. It has been observed on macOS 15 (Sequoia) and later. Only connections that the application itself opens to addresses in the local network are affected: direct connections by IP address or computer name, and connections to an RU Server in the same network. Incoming connections to the Host and connections over the Internet are not affected.

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Solution

1. Open System Settings→Privacy & Security→Local Network.
2. Find Remote Utilities Viewer (or Host, or Agent) in the list. Switch it off, then on again.
   If the application is not in the list, launch it, try to connect, and click Allow in the "would like to find and connect to devices on your local network" prompt.
3. Quit the application completely (Command+Q) and launch it again.
4. Connect again. If it still fails, restart the Mac and try once more.

See also: Can I directly connect to a remote PC using its IP address?