Macbook Air 2013 Trackpad press/click function failed

I have a very frustrating problem with the trackpad on my MacBook Air (11-inch, Mid 2013). The normal press-to-click function does not work, which also means I cannot hold down to drag-and-drop.

It started some time ago, no cause known (perhaps an automatic software update?). There is no visible sign of damage and no known incident of dirt or liquid ingress.

Everything works fine with a wired USB mouse.

I just replaced the trackpad and that did not solve the problem (!), so it seems to be something to do with software / firmware!

Symptoms:

. I can move the pointer using the trackpad

·  I can select by tapping if I enable the ‘tap to click’ option in the system preferences (using the mouse)

·  Nothing happens with a normal press on the trackpad

·  Holding down and moving (drag) does not work  

·  It is the same with all apps

I have already:

·  Checked the software is up to date (Big Sur 11.6.2)

·  Reset the SMC

·  Reset the PRAM

·  Changed the trackpad hardware (and cleaned around it inside).

That’s about everything I could find on the Internet!

Is there a fundamental problem with Big Sur on a 2013 MacBook Air?