I'm having the same issue. I've been using Smarthings with Alfred 3 for quite some time now with no issue, however this past weekend I just upgraded to the SmartThings V2 hub and I had to re-add all my devices and set it up from scratch. At first I tried to just re-authenticate via st_login but it didn't work. So I've tried removing the workflow and reinstalling it from github. I've checked core.py info / applescript and all verified they say Alfred 3 as I see the last commit shows that these changes were in fact made to the workflow anyway.
If I remove and reinstall the workflow and login, I authenticate just fine, however I get the same "stopping embedded HTTP server" popup and nothing happens after I run st_update and st_login says I'm already authenticated and trying it again doesn't do anything either.
So I'm running Alfred 3 and verified I'm using the latest SmartThings workflow that all mention "Alfred 3" in the various files. Frankly I'm not sure what else I can do here to get it working again?