I just upgraded to PM 5.0 build 17146 (60130ac), and I'm running it on a Macbook Pro running OS X 10.11.3. Prior versions of PM were able to transmit to PhotoShelter using the concurrent uploads option.
I've attempted to switch the option on and off, and restarted PM several times. However, when uploading, I am only getting one file at a time. Likewise, there is now only one progress bar for transmitting (there used to be two separate progress bars for concurrent uploads).
Any suggestions?