Yes, this runs without problems:
sudo apt update
Hit:1 http://repo.puri.sm/pureos green InRelease
Hit:2 http://www.deb-multimedia.org stretch InRelease
Hit:4 https://repo.skype.com/deb stable InRelease
Ign:3 http://cdn-fastly.deb.debian.org/debian stretch InRelease
Hit:5 http://cdn-fastly.deb.debian.org/debian stretch Release
Reading package lists... Done
Building dependency tree
Reading state information... Done
146 packages can be upgraded. Run 'apt list --upgradable' to see them.
I have some additional URLs in my /etc/apt/sources.list (I needed those to install some non-free packages, maybe there’s the problem):
deb http://repo.puri.sm/pureos/ green main
deb-src http://repo.puri.sm/pureos/ green main
deb http://httpredir.debian.org/debian stretch main contrib non-free
deb http://www.deb-multimedia.org stretch main non-free
(but my “gstreamer1.0-plugins-bad”-problem persists even if I remove the non-free URLs).