@HufflepuffBR tnx! I added my user to the group as described in neon1ks answer on this page: http://bit.ly/2psoi7k

I did the other things written there, but instead of nvidia-361, I added nvidia-381 everywhere, as that is the version I have installed.

Still, this happens:
$ optirun gedit
[ 61.006030] [WARN]Could not open configuration file: /etc/bumblebee/bumblebee.conf
[ 61.006050] [WARN]Using default configuration

If you know what to do please let me know.