Can you try two simple free fixes? One is cover up a part of the panel. Two is make sure that the panel is not in direct sunlight by adjusting its position relative to the sun.
The sun finally came back out, so went and tried the proposed free fixes.
Tried to cover the panel and it seemed to work at first...but I couldn't quite figure out how the three IQ outputs were wired to the 24 individual cells of the panels. So moving the cover around to bring on a charge can accidentally uncover more cells connected to the active output than expected, and let the voltage jump over 5.1 V and the headset block the charge.
And unfortunately, I also found out the IQ Ports have some capacitance, so you'd either have to wait for a while for the charge to dissipate and the voltage to drop back below 5.1V or plug in another load to discharge it. And same with the headset itself, once it has blocked the charge, you have to wait for a while before it accepts to charge again at a lower voltage.
So the regulator may not be free, but seems to somewhat simplify the process.
Was good fun though, thanks. Learned a few things in the process.