Hello,
If this is to any help, I have the same problem with DVBSky T680CI USB.
Upgrading Ubuntu from 15.04 to 15.10 made MythTv stop working (No Lock).
None of the kernels of Ubuntu 15.10 so far works, ie, 4.2.0-17...23.
Booting with 3.19.0-32 makes it works again.
I tried w_scan, and with 4.2.0 it finds nothing, with 3.19 no problem.
I found lots of lines like this in mythbackend.log, it might point in the direction of the problem:
mythbackend[3707]: E SignalMonitor recorders/dvbchannel.cpp:1026 (GetSignalStrength) DVBChan[5](/dev/dvb/adapter0/frontend0): Getting Frontend signal strength failed.#012#011#011#011eno: Operation not supported (95)
Best regards,
Olle Olsson