I’m having the same problem with a newly imported site. Using 2.0.7 pre-release, macOS 10.12.6, “Preferred” environment. Restarting the site does not fix it. (Also, unrelated, the reason I’m running 2.0.7 is that I had to upgrade from 2.0.6 to even import my site, in 2.0.6 it would just hang on “Extracting archive”.)
I also tried changing write permissions on the upload directory/sub-dirs to be writeable by all, and it still didn’t work.