the wifi is off
I use a GPS which is seen as a serial USB port.
The serial port is like a modem ttyACM0
usb 1-1: new full-speed USB device number 4 using dwc_otg
cdc_acm 1-1:1.0: This device cannot do calls on its own. It is not a modem.
cdc_acm 1-1:1.0: ttyACM0: USB ACM device

This works fine when i plug the GPS directly on the carambola dev kit.
for ex stty -F /dev/ttyACM0 answers immedialty

When i use a hub (powered or not) the command
stty -F /dev/ttyACM0 hangs up and the carambola finish to reboot.

I have tested the gps on diffrent hardware with x86, raspberry PI, with or without openwrt and with different usb hub. All is correct.

Only carambola with usb hub does not work.
I have the same problem with the current trunk of openwrt with
Target System (Ralink RT288x/RT3xxx) │ │
Subtarget (RT305x based boards)
openwrt-ramips-rt305x-carambola-squashfs-sysupgrade.bin

Where to investigate and which test to do ?

Hmm ... I did encounter a similar issue while using a sound device connected using USB.
It would work fine when connected directly, but only would produce noise when connected using a hub.

I figured out that rebooting the carambola once after powering up would solve the issue.

Seems like the carambola doesn't deliver the power needed for starting the device behind a hub if it's started cold.
I also remember a thread in which it is suggested to add an extra capacitor on the carambola's USB power rail.

The additionnal capacitor does not solve the problem.

To answer to myself, with:
vi /etc/modules.d/54-usb-rt305x-dwc_otg
dwc_otg speed=1

it's works,

but the usb run in Full Speed mode (12 Mb/s) instead of High Speed mode (480 Mb/s)