I uploaded the default keymap for the first time. The left hand keyboard flashes during upload (but not the right), and initially everything is fine, but when I press the LED key, things go awry. The first time, the right hand keyboard stopped working. I uploaded several times. Most were the same, but the most recent one both sides of the keyboard stop working after I hit the LED key.
What color are the LEDs when the keyboard stops working? Are they bright white, by any chance?
No. Generally they are off. The LED key, in this last test, was still functional, though both sides of the keyboard were otherwise not responding. I don’t know if this is a clue but the lower right key : dash, lowline and pipe, has the blue LED on under it. No other key is lit.
As I test further, a little later the LED key also becomes unresponsive, all but 1 LED off.
It turns out I was using version 1.93beta. I rolled back to the last release version 1.23, and the problems all went away.