Problem fixed! Problem cause: days ago, I tried to insert the Real-Time-Clock battery upside down. It made contact, but wouldn’t seat fully (of course). I removed the battery, but DIDN’T insert it at all. The Mayfly never worked again (I tried for days) until I gave up, posted my plea for help, and happened to install the RTC battery (the correct way). The Mayfly LEDs started to cheerfully blink. Dunno why this caused a problem, and I hope this helps someone else.