Cannot switch Bluetooth on and off from GUI or with function key, however it works from the terminal

Today I've realized that I can't switch bluetooth on and off

(i) from the settings GUI with the slider (the slider moves, yet there is no menu available for selecting detected devices) and
(ii) by pressing the respective function key on my Lenovo Thinkpad T470 running Ubuntu 18.04.

Interestingly, it has worked fine previously and I can switch bluetooth on and off from the command line with sudo rfkill unblock all and rfkill block bluetooth, respectively.

What is the problem here?

Additional info:

• rfkill list returns that bluetooth is soft- but not hard blocked.