Hi,
You cannot manipulate the Web Player transcoding options. The quality pull down works but you cannot select them.
You can also not select software or any of the deinterlacer options.
I am using version 2017.08.02.1643 on Synology DS916+ linux.
Sounds like a browser issue. What browser/device are you using?
Firefox v51.0.1 (32-bit) on Windows 7.
Must be browser. It is working with Chrome 59.0.3071.115.
Could you try a newer Firefox?