ObinsKit is occasionally unable to contact the firmware server at https://releases.obins.net. When this happens, you can't use ObinsKit to do any firmware upgrades or downgrades.
This repository contains some firmware .bin files just in case you need them for your C15 or C18 revision Anne Pro 2.
See the Open Anne Pro project for the tool annepro2_tools (Win/Mac/LINUX) needed to flash these .bin files directly to your Anne Pro 2 keyboard over a USB cable (with Bluetooth turned off using the hardware switch).