Since a few weeks I experience the same issue: a persistent update error. Automatic or manual does not make a difference. Fortunately it does seem to update, regardless of the error. My system is therefore up to date. Today I updated to the January 2018 update, but the error persists.
Thank you for your response. I already have the following in sources.list:
deb http://ftp.debian.org/debian stretch main contrib non-free
deb http://ftp.debian.org/debian/ stretch-updates main contrib non-free
deb http://security.debian.org/ stretch/updates main contrib non-free
deb http://apt.osmc.tv stretch main
I don’t really understand what moonlight has to do with it. I don’t use moonlight.