It seems to be recent updates from almost all mods, for example if you try to download Interstellar Extended version 1.8.20 or below (from the Changelog tab) it downloads correctly, but any version after that gives an "Oops! This link appears to be broken" message (on Internet Explorer). KSP Mod Admin program can't download these from spacedock either (checked for Connected Living Space, Engine Lighting, Engineering Tech Tree, EVA Fuel and obviously Interstellar Extended). Perhaps all new updates are failing past a certain date.