To resolve the issue of the BOO app freezing at the opening or login screen, follow these detailed steps:

1. Restart the App
- Close the BOO app completely. On iPhones, swipe up from the bottom of the screen and pause in the middle to access the app switcher. Swipe away the BOO app to close it. Then, reopen the app to see if it works properly.

2. Check Your Internet Connection
- Ensure that you are connected to a stable internet connection, whether wifi or cellular. You can toggle your wifi off and on, or switch to a different network to rule out connection issues.

3. Update the App
- Visit the App Store and check for any available updates for the BOO app. Updating can often resolve bugs that lead to freezing or crashing issues.

4. Clear Cache (if possible)
- While iOS does not provide a direct way to clear cache for individual apps, sometimes uninstalling and reinstalling the app can serve a similar purpose.

5. Reinstall the App
- If the app remains unresponsive, uninstall it by pressing and holding the app icon, tapping "Remove App" and then choosing "Delete App". After that, go to the App Store and reinstall BOO.

6. Restart Your iPhone
- A simple restart of your device can sometimes clear minor software glitches that may be affecting the app's functionality. You can restart your iPhone by holding the side button and either volume button until you see the power off slider.

7. Contact Support
- If the issue still persists after trying all the above solutions, consider contacting BOO’s support team for further assistance. They may provide specific troubleshooting steps based on your situation.

These steps have been commonly recommended in discussions regarding app issues similar to yours - learn more - learn more - learn more.