Slack made some changes to their permission model and I had to make some changes to SharedBox. Took some days of back and forth to finally got changes approved. In those few days, new users couldn't signup to the service. Coincidentally, DenseDiscovery featured SharedBox in one of the days. Google analytics showed about 300 visits that I apparently lost.
Good news, things are fine now. By the way, wrote a passport strategy for the OAuth v2 flow https://github.com/kehers/passport-slack