Cannot log in / specify discovery addresses

I have just installed Skype for Business 16.1.456 on my MBP running MacOS Sierra, and cannot log in.

(This is a fresh install, I have not run Skype for Business on the Mac earlier.)

On my Windows version of Skype for Business I have manual configuration of discovery addresses (automatically added for me by my company) so I assume that the Mac login fails because these are missing. So I attempt to add them on the Mac, however:

• When I click "Advanced Options" i get a dialog box with inputs for HTTP proxy (Domain/User Name/Password), Internal Discovery Address and External Discovery Address.
• The last two are disabled and the checkbox Auto-Detect Server is selected.
• If I uncheck Auto-Detect Server the two Discovery Address inputs become editable, but at the same time the Save button is disabled so I cannot save my manually configured discover address values.

See the attached screenshot for illustration of the problem.

Any ideas?