@Pierre, glad to hear it worked. Well, 4.5V is not unusual, but I thought 2.8V is a bit on the low side. Because that voltage comes from the 3.3V linear regulator, unless if the circuit is drawing a lot of current, it should be pretty close to 3.3V. If you want to test further, I suggest you unplug 24VAC, but instead power the circuit with a USB cable, and retest the VCC-GND voltage. See if it’s close to 3.3V. It’s just my precaution — I am pretty sensitive to these numbers, and I feel 2.8V is a bit unusual.