Hi there! I understand that your Polar Flow app gets stuck on the loading screen during sign-in, but you're able to connect to Polar Beat just fine. Let’s see what we can do to get you signed in smoothly.

To start, could you tell me if you're using a browser or the Polar Flow desktop app on your Windows 10 device? Also, have you tried any troubleshooting steps so far, like clearing cache or reinstalling the app?

In the meantime, here’s a step-by-step approach that usually helps with app loading issues during sign-in:

1. Check Internet Connection: Even if Polar Beat connects fine, ensure your Windows device has a stable internet connection.

2. Clear Cache and Cookies (If using a browser):
- For WebKit browsers, go to the browser settings and clear cache and cookies.
- Then, restart the browser and try signing in again.

3. Check Firewall and Antivirus:
- Sometimes, Windows firewall or antivirus software can block the Flow app’s connection.
- Temporarily disable them and try signing in again.
- If this works, add Flow app to allowed programs in firewall/antivirus settings.

4. Try a Different Browser:
- Since you’re on Windows 10 using a WebKit browser, try signing in with another browser like Chrome, Edge, or Firefox to see if the issue is browser-specific.

5. Sign Out and Reinstall the App (if using Flow desktop app):
- Sign out if possible.
- Uninstall the Flow app.
- Restart your computer.
- Download and install the latest version from the official Polar website.

6. Check for Server Issues:
- Sometimes Polar Flow servers may be temporarily experiencing issues.
- You can check status on Polar’s support channels or social media.

If these steps don't resolve the problem, we can explore more advanced settings or account-specific checks.

Would you be able to try these and let me know how it goes? I’m here to help further!