if you are running < 6.10 the lines are added to qemu in the hooks dir.
it should have the code inserted near the top.
#begin USB_MANAGER\nif ($argv[2] == 'prepare' || $argv[2] == 'stopped'){ shell_exec("/usr/local/emhttp/plugins/usb_manager/scripts/rc.usb_manager vm_action '{$argv[1]}' {$argv[2]} {$argv[3]} {$argv[4]} >/dev/null 2>&1 & disown") ;
}
#end USB_MANAGER
Also check the permission on the file that it is execurtable. ls -l you have the x in the flags.
can you also check usb.ini in /usr/local/emhttp/state/
Entries should look similar to this
[001/111]
ishub = ""
connected = ""
parents = "1-9,usb1,0000:00:14.0,pci0000:00"
bus = 001
dev = 111
ID_VENDOR_FROM_DATABASE = "Dresden Elektronik"
ID_VENDOR_ID = "1cf1"
ID_MODEL = "ConBee_II"
ID_MODEL_ID = 0030
USBPort = "1-9.3"
class = "interface"
ID_SERIAL = "dresden_elektronik_ingenieurtechnik_GmbH_ConBee_II_DE"
isSerial = "1"
isSerialPath = "usb-dresden_elektronik_ingenieurtechnik_GmbH_ConBee_II_DE-if00"
bNumInterfaces = 2
isSerialDevPath = "/dev/ttyACM1"