3DTouch sensor fails after probe deployment and triggering
After troubleshooting I have managed to fix the sensor problems. 

(Please take note of the update at the end of the answer below as the problem started again shortly after the so-called fix...)

Digging into several posts I found this post that hints to a solution:



After readjusting, the sensor worked and I was able to produce a print, but the next day it failed again... What I found is that the bed and extruder are NOT shut off while probing although that should have been enabled in Marlin 1.1.8. I assumed that this de-powering is only during the actual probe lowering, so very short. So I figured that this might be an EMI problem. In a last resort I updated to Marlin 1.1.9 which has the functionality to shut the power off for heater and bed and everything now seems to work perfectly. I now see that the de-powering is a bit longer and in fact the extruder temperature drops slightly, this has never been seen with the previous firmware version.



Update:
Not long after the "update"/"fix" the problem manifested itself again... Further troubleshooting revealed that it was a bad connection of the connectors of the sensor that got temporarily "fixed" as the cables had been fiddled with during the firmware update.