This is purely a Steam workshop issue because I've yet to do anything from Nexus.
I know this because I recently upgraded my computer and I've had a lot of installing to do, and Skyrim has been on my list but this error (In thread title) has gotten in my way. The game loads, but literally none of the ~68 Workshop Mods will load with it. Well, all but about five. It just gets to that error message and gives up, doesn't even bother trying to load the rest.
Not sure what this error means.