Hi! I understand you're having trouble updating the firmware through the ViCare app on your Mac with OS X 10.15.7. To get a better grasp, may I ask if you're seeing any specific error messages or codes when the update fails? Also, are you updating firmware on your Viessmann device itself or through the app only?

Meanwhile, here are some steps to check and try:

1. Check App Version and OS Compatibility:
Make sure your ViCare app version is up to date and compatible with OS X 10.15.7. Sometimes older operating systems may restrict app functions. If possible, update your device or ensure the ViCare app is the latest release.

2. Restart the ViCare app and your device:
Close the app completely, then restart your Mac. This can clear temporary glitches that interfere with updates.

3. Verify Network Connectivity:
Firmware updates require a stable internet connection. Confirm your Mac is connected to WiFi and has a reliable internet connection.

4. Allow Remote Software Updates:
Open the ViCare app, navigate to Settings > General, and check that the “Allow remote software updates” switch is activated. Without this, firmware updates may be blocked.

5. Reset and Reconnect Devices:
If you have access to the physical control module (like Vitoconnect), try pressing its reset button for a few seconds, then re-establish the connection in the app (Settings > Devices > Communication module > WiFi settings).

6. Wait Time if Errors Occur:
If you have seen error codes like E10 or E12 during the update, sometimes the system needs at least 90 minutes for the firmware on the control unit to stabilize before retrying the update, as per official guidance.

If you have tried any of these already or if different error messages show up, please let me know. That'll help us pinpoint the problem and find a suitable solution for you!

Are you satisfied with me starting here?