I'd recommend taking this to the Vortex Support Forum. If there are no issues downloading through other means, something is up with Vortex, rather than Skyrim. Plus, that section's pretty active. Just to be sure, check your settings, downloads, and make sure the option that says "Handle [Download Button] with Vortex" is enabled. Seeing something about fixing it on Chrome(?), if you use Chrome, close it out, and click that in Vortex.