Skype for Business: Others can't hear me at all

No one can hear me when I make or take a call in Skype for Business, no matter what input (microphone) source - internal or headset - I use on my iMac. (I can hear others fine through any output.)

I have tried:

- Toggling the input and output sources in both System Preferences/Audio as well as the audio preferences in Skype for business.

- Used the button "clear audio preferences" in Skype.

- Tried to erase Skype for Business from the system (don't know if I caught everything though)m reinstalled it.

- Restart the system

- plug/unplug the headset

- playing with diverse buttons at the headset

- unmute/mute buttons in Skype, of course ;-)

Counter check:

- Any of the above input sources work fine with e.g. FaceTime Audio and Video on my iMac. No issues.

- The same setup works fine on my MacBook 12" Retina (early 2016) running Mojave. However, I need to have this on my main machine, my, iMac as well.

System:

I am running Office 365, latest updates (including Skype for Business Mojave update from today), running macOS Mojave on an fully functioning iMac 5K Retina (late 2014). Using both internal mike/output as well as a Plantronics 315.1 headset. Plantronics Hub software installed, latest firmware applied today.

Are there hidden preference, extensions that "globally" allow/disallow microphone input for Skype for Business, aside from the preferences in Skype or macOS? What am I missing?

Thanks for your help!