As no trick mentioned here worked ...
I made a fresh install and updated everything except the kernel (which remains at 4.15.0-20). And my Lenovo T 520 works again.
So indeed kernel 4.15.0-24 seems the culprit. No glue why booting to previous installed kernels doesn't work either as soon 4.15.0-24 is installed.
As no trick mentioned here worked ...
I made a fresh install and updated everything except the kernel (which remains at 4.15.0-20). And my Lenovo T 520 works again.
So indeed kernel 4.15.0-24 seems the culprit. No glue why booting to previous installed kernels doesn't work either as soon 4.15.0-24 is installed.