These days when I am using the companies UMTS on my MacBook using a USB UMTS stick (similar to http://shop.vodafone.de/Shop/datenprodukt/vodafone-usb-stick-k3765-hv/) I get from time to time an error from the Vodafone Mobile Broadband application saying, that it could not connect to the internet (and "please restart the application", but this does not help).
What helped are restart the netmgrd process, it is done with following commands in the terminal:

• first get the process id with the "ps" command:
  

ps -Af | grep "netmgr"

    0    29     1   0   0:57.89 ??         1:44.15 /usr/sbin/nmnetmgrd -T 86400

  501  6387  6161   0   0:00.00 ttys005    0:00.00 grep netmgr

in our case that is the value in the second column "29"

• then kill the process with:
  

sudo kill -KILL 29

After that the "nmnetmgr" restarts automatically and pressing the "Connect" button connects again to the internet.