If your version of policykit-1 is policykit-1 (0.105-15) - you do not need to read the rest of this message.
For a very, very small segment of Sparky users, they might have a specialized version of policykit-1 that has stayed around awhile as certain updates from experimental have not crossed over to unstable and testing. The computers affected are ones that have been running LXQT for more than 6 months and that received LXQT packages from the Siduction Repositories.
This will probably not be the case unless you have
deb http://packages.siduction.org/lxqt unstable main
in your /etc/apt/sourceslist.d/siduction.list
The specific program is policykit-1 Version 0.112/5/3/1_really-0.105-8
Better and more specific information can be found at
http://forum.siduction.org/index.php?topic=6226.msg51228#msg51228
I am not telling or even suggesting what you should do about this. I did do the
apt-get install $(dpkg -l | awk '/0.112-5.3.1~really-0.105-8/ {print $2"=0.105-15"}') # downgrade packages
on two computers as I have benefited in the past from Musca's knowledge. It is your computer and your operating system. Do as you see fit after investigating.
Background info on policykit-1 in Testing-Stretch-Sparky is at
https://packages.debian.org/stretch/policykit-1