I am running 15.10 on a Lenovo ThinkPad R61 and have been running Ubuntu since 8.04 on this machine. Since upgrading to 15.04 I have serious trouble with mobile broadband connections via USB modems.
While nm-applet says that a connection was established, there is actually no connection. One needs to repeat the 'connect to...' action several times until eventually a working connection becomes established. Once the connection is established, it remains stable as long as the current user is in an active session.
I have no clue where to start looking for system messages that could help further. On booting I occasionally see 'Internal Errors detected' messages mostly showing crashes of a package. Right now it says the urfkill package has crashed. Could there be a connection to the network connection issues?
I am happy to provide you any logfile output that could help. However, I did not want to flood my posting with potentially useless logfile copies. What can be done to enable mobile broadband to connect reliable and on the first trial?