14.04: Modem Manager Always Crashes

I have just installed Ubuntu 14.04 (64-bit) and when I tried to connect to Internet via USB modem (Sierra 308), modem manager always crashes. The USB modem worked well in Ubuntu 12.04 that I used before. Crash report is attached here.

Here I also include an excerpt of Syslog:

Jul 3 22:27:25 reinaldo-home ModemManager[4142]: <info> ModemManager (version 1.0.0) starting...
Jul 3 22:27:25 reinaldo-home NetworkManager[725]: <info> Unmanaged Device found; state CONNECTED forced. (see http://bugs.launchpad.net/bugs/191889)
Jul 3 22:27:25 reinaldo-home NetworkManager[725]: <info> Unmanaged Device found; state CONNECTED forced. (see http://bugs.launchpad.net/bugs/191889)
Jul 3 22:27:25 reinaldo-home NetworkManager[725]: <info> NetworkManager state is now CONNECTED_GLOBAL
Jul 3 22:27:25 reinaldo-home NetworkManager[725]: <info> Unmanaged Device found; state CONNECTED forced. (see http://bugs.launchpad.net/bugs/191889)
Jul 3 22:27:25 reinaldo-home NetworkManager[725]: <warn> (ttyUSB2) failed to connect modem: Message did not receive a reply (timeout by message bus)
Jul 3 22:27:25 reinaldo-home NetworkManager[725]: nm_device_get_state: assertion 'NM_IS_DEVICE (device)' failed
Jul 3 22:27:25 reinaldo-home NetworkManager[725]: modem_prepare_result: assertion 'state == NM_DEVICE_STATE_PREPARE' failed
Jul 3 22:27:25 reinaldo-home NetworkManager[725]: <info> ModemManager disappeared from bus
Jul 3 22:27:25 reinaldo-home NetworkManager[725]: <warn> (ttyUSB2) failed to disconnect modem: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name :1.113 was not provided by any .service files
Jul 3 22:27:23 reinaldo-home whoopsie[1055]: message repeated 4 times: [ offline]
Jul 3 22:27:25 reinaldo-home whoopsie[1055]: Could not determine if this is a default route:
Jul 3 22:27:25 reinaldo-home whoopsie[1055]: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: Method "Get" with signature "ss" on interface "org.freedesktop.DBus.Properties" doesn't exist#012
Jul 3 22:27:25 reinaldo-home whoopsie[1055]: Could not determine if this is a default route:
Jul 3 22:27:25 reinaldo-home whoopsie[1055]: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: Method "Get" with signature "ss" on interface "org.freedesktop.DBus.Properties" doesn't exist#012
Jul 3 22:27:25 reinaldo-home whoopsie[1055]: offline
Jul 3 22:27:25 reinaldo-home NetworkManager[725]: <info> ModemManager available in the bus
Jul 3 22:27:27 reinaldo-home ModemManager[4142]: <warn> Couldn't find support for device at '/sys/devices/pci0000:00/0000:00:1c.4/0000:02:00.0': not supported by any plugin
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.626433] usb 2-1.5: new high-speed USB device number 14 using ehci-pci
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.720098] usb 2-1.5: config 1 has an invalid interface number: 9 but max is 5
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.720104] usb 2-1.5: config 1 has an invalid interface number: 7 but max is 5
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.720107] usb 2-1.5: config 1 has no interface number 2
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.720109] usb 2-1.5: config 1 has no interface number 5
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.721596] usb 2-1.5: New USB device found, idVendor=0f3d, idProduct=68a3
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.721600] usb 2-1.5: New USB device strings: Mfr=3, Product=2, SerialNumber=4
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.721603] usb 2-1.5: Product: USB 308
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.721606] usb 2-1.5: Manufacturer: Sierra Wireless, Incorporated
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.721608] usb 2-1.5: SerialNumber: 353354046068858
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.725157] sierra 2-1.5:1.0: Sierra USB modem converter detected
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.726746] usb 2-1.5: Sierra USB modem converter now attached to ttyUSB0
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.726828] sierra 2-1.5:1.1: Sierra USB modem converter detected
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.727244] usb 2-1.5: Sierra USB modem converter now attached to ttyUSB1
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.727324] sierra 2-1.5:1.3: Sierra USB modem converter detected
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.727696] usb 2-1.5: Sierra USB modem converter now attached to ttyUSB2
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.727772] sierra 2-1.5:1.4: Sierra USB modem converter detected
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.728190] usb 2-1.5: Sierra USB modem converter now attached to ttyUSB3
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.728266] usb-storage 2-1.5:1.9: USB Mass Storage device detected
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.728482] scsi16 : usb-storage 2-1.5:1.9
Jul 3 22:27:30 reinaldo-home kernel: [ 9593.730138] sierra_net 2-1.5:1.7 wwan0: register 'sierra_net' at usb-0000:00:1d.0-1.5, Sierra Wireless USB-to-WWAN Modem, 46:e0:69:42:09:07
Jul 3 22:27:30 reinaldo-home mtp-probe: checking bus 2, device 14: "/sys/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.5"
Jul 3 22:27:30 reinaldo-home mtp-probe: bus: 2, device: 14 was not an MTP device
Jul 3 22:27:30 reinaldo-home ModemManager[4142]: <warn> (ttyUSB0): port attributes not fully set
Jul 3 22:27:30 reinaldo-home NetworkManager[725]: SCPlugin-Ifupdown: devices added (path: /sys/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.5/2-1.5:1.7/net/wwan0, iface: wwan0)
Jul 3 22:27:30 reinaldo-home NetworkManager[725]: SCPlugin-Ifupdown: device added (path: /sys/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.5/2-1.5:1.7/net/wwan0, iface: wwan0): no ifupdown configuration found.
Jul 3 22:27:30 reinaldo-home ModemManager[4142]: <warn> (ttyUSB1): port attributes not fully set
Jul 3 22:27:30 reinaldo-home ModemManager[4142]: <warn> (ttyUSB2): port attributes not fully set
Jul 3 22:27:30 reinaldo-home ModemManager[4142]: <warn> (ttyUSB3): port attributes not fully set
Jul 3 22:27:31 reinaldo-home kernel: [ 9594.728369] scsi 16:0:0:0: Direct-Access SWI SD Card 2.31 PQ: 0 ANSI: 2
Jul 3 22:27:31 reinaldo-home kernel: [ 9594.728720] sd 16:0:0:0: Attached scsi generic sg2 type 0
Jul 3 22:27:31 reinaldo-home kernel: [ 9594.730950] sd 16:0:0:0: [sdb] Attached SCSI removable disk
Jul 3 22:27:41 reinaldo-home ModemManager[4142]: <info> Creating modem with plugin 'Sierra' and '5' ports
Jul 3 22:27:41 reinaldo-home ModemManager[4142]: <warn> Could not grab port (tty/ttyUSB0): 'Cannot add port 'tty/ttyUSB0', unhandled serial type'
Jul 3 22:27:41 reinaldo-home ModemManager[4142]: <warn> (ttyUSB2): port attributes not fully set
Jul 3 22:27:41 reinaldo-home ModemManager[4142]: <info> Modem for device at '/sys/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.5' successfully created
Jul 3 22:27:42 reinaldo-home ModemManager[4142]: <warn> Couldn't load unlock retries: 'Unknown error'
Jul 3 22:27:49 reinaldo-home ModemManager[4142]: <warn> couldn't load Operator identifier: 'SIM failed to handle CRSM request (sw1 103 sw2 0)'
Jul 3 22:27:50 reinaldo-home ModemManager[4142]: <warn> couldn't load list of Own Numbers: 'Failed to parse NV MDN command result: -17'
Jul 3 22:27:53 reinaldo-home ModemManager[4142]: <info> Modem: state changed (unknown -> disabled)
Jul 3 22:27:53 reinaldo-home NetworkManager[725]: <warn> (ttyUSB2): failed to look up interface index
Jul 3 22:27:53 reinaldo-home NetworkManager[725]: <info> WWAN now disabled by management service
Jul 3 22:27:53 reinaldo-home NetworkManager[725]: <info> (ttyUSB2): new Broadband device (driver: 'sierra, sierra_net' ifindex: 0)
Jul 3 22:27:53 reinaldo-home NetworkManager[725]: <info> (ttyUSB2): exported as /org/freedesktop/NetworkManager/Devices/10
Jul 3 22:27:53 reinaldo-home NetworkManager[725]: <info> (ttyUSB2): device state change: unmanaged -> unavailable (reason 'managed') [10 20 2]
Jul 3 22:27:53 reinaldo-home NetworkManager[725]: <info> (ttyUSB2): deactivating device (reason 'managed') [2]
Jul 3 22:27:53 reinaldo-home NetworkManager[725]: <info> NetworkManager state is now DISCONNECTED
Jul 3 22:27:53 reinaldo-home NetworkManager[725]: <info> (ttyUSB2): device state change: unavailable -> disconnected (reason 'none') [20 30 0]
Jul 3 22:27:56 reinaldo-home AptDaemon: INFO: Quitting due to inactivity
Jul 3 22:27:56 reinaldo-home AptDaemon: INFO: Quitting was requested
Jul 3 22:28:01 reinaldo-home NetworkManager[725]: <info> disable requested (sleeping: no enabled: yes)
Jul 3 22:28:01 reinaldo-home NetworkManager[725]: <info> sleeping or disabling...
Jul 3 22:28:01 reinaldo-home NetworkManager[725]: <info> (eth0): device state change: unavailable -> unmanaged (reason 'sleeping') [20 10 37]
Jul 3 22:28:01 reinaldo-home NetworkManager[725]: <info> (eth0): cleaning up...
Jul 3 22:28:01 reinaldo-home NetworkManager[725]: <info> (eth0): taking down device.
Jul 3 22:28:01 reinaldo-home NetworkManager[725]: <info> NetworkManager state is now ASLEEP
Jul 3 22:28:01 reinaldo-home NetworkManager[725]: <info> (ttyUSB2): device state change: disconnected -> unmanaged (reason 'sleeping') [30 10 37]
Jul 3 22:28:01 reinaldo-home NetworkManager[725]: <info> (ttyUSB2): cleaning up...
Jul 3 22:28:01 reinaldo-home NetworkManager[725]: <info> (ttyUSB2): taking down device.

Could you please tell me how to fix this issue?