Acabo de experimentar este mismo problema después de actualizar a 16.04.1 desde 14.04 (bajo el cual la conexión USB funcionó bien durante años).
Según el registro del sistema, parece que es causado por un conflicto con ModemManager.
Aug 25 13:12:24 outback NetworkManager[2543]: <info> [1472145144.1334] device (enp0s20u2): Activation: successful, device activated.
[...eliding log entries related to nm-dispatcher services]
Aug 25 13:12:38 outback ModemManager[2503]: <info> Creating modem with plugin 'Generic' and '2' ports
Aug 25 13:12:38 outback ModemManager[2503]: <info> Modem for device at '/sys/devices/pci0000:00/0000:00:14.0/usb3/3-2' successfully created
Aug 25 13:12:41 outback ModemManager[2503]: <warn> couldn't load Manufacturer: 'Serial command timed out'
Aug 25 13:12:44 outback ModemManager[2503]: <warn> couldn't load Model: 'Serial command timed out'
Aug 25 13:12:47 outback ModemManager[2503]: <warn> couldn't load Revision: 'Serial command timed out'
Aug 25 13:12:50 outback ModemManager[2503]: <warn> couldn't load Equipment Identifier: 'Serial command timed out'
Aug 25 13:12:50 outback ModemManager[2503]: <warn> couldn't load Operator name: 'SIM failed to handle CRSM request (sw1 106 sw2 130)'
Aug 25 13:12:54 outback ModemManager[2503]: <warn> couldn't load IMEI: 'Serial command timed out'
Aug 25 13:12:54 outback ModemManager[2503]: <info> Modem: state changed (unknown -> disabled)
Aug 25 13:12:54 outback NetworkManager[2543]: <info> [1472145174.3879] device (enp0s20u2): state change: activated -> deactivating (reason 'connection-removed') [100 110 38]
Aug 25 13:12:54 outback NetworkManager[2543]: <info> [1472145174.3881] manager: NetworkManager state is now DISCONNECTING
Aug 25 13:12:54 outback NetworkManager[2543]: <info> [1472145174.4362] dns-mgr: Writing DNS information to /sbin/resolvconf
Aug 25 13:12:54 outback dnsmasq[4348]: setting upstream servers from DBus
Aug 25 13:12:54 outback NetworkManager[2543]: <info> [1472145174.4430] manager: (ttyACM0): new Broadband device (/org/freedesktop/NetworkManager/Devices/4)
Aug 25 13:12:54 outback NetworkManager[2543]: <info> [1472145174.4441] device (ttyACM0): state change: unmanaged -> unavailable (reason 'managed') [10 20 2]
Aug 25 13:12:54 outback NetworkManager[2543]: <info> [1472145174.4445] device (ttyACM0): modem state 'enabling'
Aug 25 13:12:54 outback NetworkManager[2543]: <info> [1472145174.4457] device (enp0s20u2): state change: deactivating -> disconnected (reason 'connection-removed') [110 30 38]
Inhabilité ModemManager a través del comando:
sudo systemctl disable ModemManager.service
Y luego reiniciado. Mis sesiones de conexión USB posteriores han sido estables desde entonces.
Estoy seguro de que hay una manera de configurar ModemManager correctamente para mi teléfono (un LG G3), pero actualmente no necesito un módem en serie, por lo que este método funciona para mí.