I was struggling with the same issue when I was trying to change profiles via URL command. I ended up going into Blue Iris Settings > Web Server > Advanced (bottom of web server tab). Then I unchecked "Use Secure Session Keys and Login Page". The URL command works and the only difference I see when using UI3 is that a login pop-up appears instead of the login screen. I found this through trial and error after playing with several settings, I *think* this was the one setting that made it work, but it could be a combination of something else I also did.