I tried to launch Substance Designer tonight, but when it didn't launch initially I remembered that the 5.5 update had been released within the previous 24-48 hours. So I opened up Steam to see if the update was downloading and saw there were a few updates in the Queue. A DOTA2 update and something else downloaded and installed perfectly fine, but the Substance Designer update could not be started.
Clicking the "Download Now" button moves it into the Queued position for less than a second, then moves it back down into the list. This also forces the "Scheduled For" time to update (it updates on its own as well) to 5 minutes after the current time.
Clicking "Launch" gives the error: "An erro