I'm having almost the exact same problem with a 1166pro75. I've been trying to update the firmware but get an unhelpful "update faild" error
You might be trying to downgrade the firmware or the firmware you downloaded has a different extension, check carefully if the firmware you are trying to flash is not corrupted.
Anyway, there is a video guide on how to properly upgrade the Avalon unit you can follow the video from this link below. The audio is Chinese but it has a subtitle.
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User guide for AvalonMiner - firmware upgrade