Flashing new firmware makes my keyboards unresponsive

I’ve been stuck on my current custom firmware for the better part of a year. It is the only thing that works on both my Model 01s. Changing the sketch code and flashing makes the keyboards unresponsive. Since my custom firmware works, I’m confident it’s not a hardware issue. The only thing I have left of my current firmware is the hex file (the working sketch is lost due to my code changes), which I flash using the terminal/cmd.exe whenever my attempts to flash a new firmware fail.

I’ve tried doing everything from scratch, following the step-by-step guide. Even the default sketch/firmware results in an unresponsive keyboard. I’ve tried this on Windows 7 and Windows 10 on both my keyboards, and on different computers.

Changing the firmware used to work in the past (before the API changes I think, if that matters), though I’ve had similar issues with unresponsive keyboards if I didn’t comment out some lines of code (which I tried with the latest default firmware as well, but it didn’t help this time). It’s all so very strange.

I would appreciate it if somebody could help me out.

Are you pulling the code from git or from the Arudino IDE’s Boards manager? If you’re using one, can you try the other?