Unfortunately, after updating from version 4.4 to 4.9, the panel got stuck in a boot loop. Is there a procedure to fix this? The logo appears for 2 seconds along with the small text "installing Sonoff update , and flashes. I have ADB access, but I don’t know what I can do.
Try this. It once worked for me in a very similar situation. Disconnect the panel from power for at least a minute. Restore the power. Afterwards it should regain consciousness. 
Thanks for the tip. It didn’t help, though.
If the device is still unable to boot into the system, you can try using the disaster recovery mode.
You mentioned that ADB had already been enabled. Could you let me know what operation you performed before this issue occurred?
Disaster recovery steps:
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Power on the device for the first time. Wait until the SONOFF logo has fully loaded, then power it off.
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Power it on again. Wait until the SONOFF logo has fully loaded, then power it off.
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Repeat the same process for the third power-on.
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Repeat it again for the fourth power-on.
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Repeat it again for the fifth power-on.
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On the sixth power-on, the device should successfully reset and enter the initial setup process. You will need to configure the language, region, and other settings again.
Note: “Wait until the logo has loaded” means you should wait until the SONOFF logo is fully displayed before disconnecting the power.
Please be careful when repeatedly powering the device on and off, and make sure to follow proper electrical safety precautions.
Have you already tried the disaster recovery procedure described above?
If possible, please record a detailed video showing the full power-off and power-on process. I can check the video to confirm whether the disaster recovery procedure was triggered successfully.
Also, one more question: have you ever manually flashed or downgraded the NSPanel Pro firmware to an older version? Since ADB access can provide this level of permission, I’d like to confirm whether that was done previously.
I once had issues with version 2.3. Back then, the 5-reset procedure helped me out. I never installed any software to downgrade the version. I recall that the resets did downgrade the version, but the OTA update fixed everything. This time, I had version 4.4 and upgraded it to 4.9. Below is a video showing the restoration procedure.
I managed to fix it by connecting via USB cable and using ADB. It turned out the problem was a 90-degree screen rotation that occurred after the first stage of the update. In keypart, you need to change {26}{smatekandroidrotation=90} to {26}{smatekandroidrotation=0} using the command: adb shell "busybox dd if=/tmp/keypart.bin of=/dev/block/by-name/keypart bs=512". Hopefully, this helps someone. This is the second time an update has broken the device. I managed to bring it back to life, but this isn’t a normal situation. I didn’t do anything unusual; I just clicked “update.” That put the panel out of commission for about a week. I hope future updates go smoothly.