Solar Power Manager and servo motor

I'm trying to make an automatic door opener for my chicken coop.

I've cabled a light sensor and a servo motor to an Arduino Nano and everything is powered up by the DfRobot solar power manager and a 18650 battery cell.

Things work perfectly independently.

I've enabled low power mode on Nano and on power manager following the wiki page.

When the servo is not connected (D9 disconnected), the nano wakes up every minute and read the value of the light sensor (it blinks briefly as expected).
When I connect the servo control pin to D9, the Nano resets and the light sensor is on forever.

Did I make something wrong with it ? What's strange is the Nano is powered up without the USB cable.

Code is available here: https://pastebin.com/2UvuRyj9