Conference room laptop would not allow sign-in for any user, as each attempt timed-out and received "can't sign in to skype for business" error.
Microsoft has a dedicated page to sign-in errors. It presents a host of troubleshooting steps:
My problem: time on laptop was out of sync with server (I had it set to manual time set). Once I pushed sync time with server slider to "on" I was able to successfully sign-in to Skype for Business desktop app.