Comment 16 for bug 138957

It seems that the problem has two parts. The first is wrong owner of /dev/input/uinput: it should be root:plugdev instead of root:root

The second part of the problem can be seen after manually changing the owner:
     sudo chown root\:plugdev /dev/input/uinput

Now after locking the screen you should see 'Password or swipe finger: ' prompt in the unlock dialog but unlocking doesn't work. Swiping finger has no effect and after entering password the dialog hangs saying 'Checking...'. I didn't wait to see if it ever finishes and killed the gnome-screensaver process from console (login didn't offer to swipe finger, only password).

I tried reproduce the above with 'gnome-screensaver --debug --no-daemon &> gs.out' and attached the output. Also, I noticed that this is not always reproducible. Sometimes swiping works in the unlock dialog and it hangs only after n-th attempt to lock/unlock the screen.