Comment 6 for bug 1656776

You're right indeed, your issue is likely different from bug 1607535, but once you install apt-transport-https you should run into bug 1607535.

That package is normally installed as a Recommends of ubuntu-standard: did you install your system without Recommends?

Going through the comments of bug 1651923's duplicates (a lot of them are wrongly marked IMHO), it appears downloads.sourceforge.net was already redirecting to HTTPS urls on 2016-11-24 (if not before, but I couldn't find evidence of it). You're having that issue since 2017-01-12 only: was it when you first tried installing this package?