They've been changing the infrastructure of the site, and I'm taking a wild guess that they're having to move the mods around to do so. The "file does not exist problem" has been on-and-off for several days. My guess is that they should be back up by later today, and if not, well, people on the forums have been saying that this maintenance will be done by Monday. I'm not sure. Yeah, Nexus, go ahead and turn off mod download-ability while I'm halfway through downloading the Beyond Boulder Dome stuff. :(