@chises Sorry it took so long to reply.
Yeah I'm not sure, I'll look into it though. I found the same issue reports you just did, and a few other issue reports in the SoundCloud extension that seem to imply that it has problems with empty lists and stuff. I don't fully understand the issues yet because I actually just now found those while looking for something else.
So as soon as I solve my current problems I'll see if there's something that can be done to make this clearer or improve it a little.
Btw, I added the SoundCloud sets support to the backend (that's my fork), but I only made sure it worked when adding and playing e.g. via MPC. I didn't do any tests against the web interface because I don't use it, so I'm actually not really sure if my set support will extend over to the web UI or if there's something else that also needs to be done (the backend API is pretty much completely undocumented -- a somewhat surprising move for a library that touts itself as extendable -- so it's hard to guess what all has to happen).
The only bits I added to get set support working are visible in the patch at https://github.com/JC3/mopidy-soundcloud/commit/5d546a584da3eb7292e9ccc915f73e5df94149d0, where I added the ability for it to see a "playlist" response then simply iterate through the tracks and add them like it would add a normal track, and that's the highest-level spot I found to do it. So I think it should work in the web interface, too, assuming calling
self.sanitize_tracks is all that needs to happen.