Hi there! I see you're running into connection errors 5001, 400, and 0 with the OneCast app on your Mac running OS X 10.15.7. I'd like to help you get this sorted out. To start, can you tell me a bit more about the situation? For example:

- When exactly do these errors appear? (e.g., when launching the app, trying to connect to your Xbox)
- Have you recently changed any network settings or updated your system or app?
- Have you tried any troubleshooting steps so far?

Knowing these details will help me guide you better. Meanwhile, I can offer some general steps to tackle these errors:

1. Check Network Connection
Ensure your Mac and your Xbox (if applicable) are on the same local network and that your internet connection is stable.

2. Restart Devices
Restart your Mac, Xbox, and your Wi-Fi router to clear out any temporary glitches.

3. Update OneCast App & macOS
Make sure your OneCast app is updated to the latest version compatible with macOS 10.15.7, and verify if any macOS updates are pending.

4. Firewall and Security Settings
Verify that your Mac's firewall or any antivirus software isn’t blocking OneCast from communicating on the network.

5. Check OneCast Permissions
Grant OneCast necessary permissions in System Preferences > Security & Privacy > Privacy (like accessibility or network access if applicable).

6. Reinstall OneCast
If the problem persists, try uninstalling OneCast completely and reinstalling it to fix possible corrupted files.

If these initial steps don’t help, since error 5001 can sometimes relate to internal app connectivity or a server-related issue, I can check for any known outages or additional fixes.

Does this sound like a good place to start? Also, feel free to share exactly when the error codes show up and any messages you see—this will help me narrow down the cause.