That's very weird...
Although you made the bridges, don't trust them, breadboards like to fail a lot, ensure GND is properly conected, some breadboards are splitted internally.
Connect both sides of the DMM at the stm32 board pins.
If everything is ok, the only thing I can think of is the stm32 resetting randomly.
Try blinking PC13, it's the onboard led, should be a consistent on/off, you'll easily see how it toggles and there's no way you can mess the connection.
Set PC13 as output, no pull, name it LED, then:
while(1){
HAL_GPIO_Toggle(LED_GPIO_Port, LED_Pin);
HAL_Delay(1000);
}
Is it a 401 or a 411?
Attach your project as-is and il'll test it.
The gnd wire looks suspicious...