Hi,
I have try the S110 v7.1.0 with DFU (build with SDK 6.x).
And the DFU don't work...
The DFU from SDK 6.x works with S110 v7.1.0. I tried with Master Control Panel on a computer with DFU from both SDK 6.0.0 and 6.1.0. The DFU was compiled from a fresh copy of the SDK.
Please update your question with more info on what platform you are using, if you have changed the code in any ways, and what you are trying to update (Softdevice, app or bootloader)