thanks for the tips,… gonna tell you my win-win-story 
ok, just installed this recommended app … and tried all sub-tools,
accelerometer shows nothing,
gyroscope shows nothing, …
only LIGHT shows smthng, 39,0 lux - but never changes (not in dark room, not by changing screen brightness) … only here the “+”-button in the app works… ;/
not that happy with that outcome… and thought maybe this app is broken, so i downloaded
sensors multitool from wered software
and sensoren werkzeugkasten from exa tools
both showed the same as the first app - nothing.
so i rebooted my fairphone 3, et voila, it ‘suddenly’ works…
android seems more and more like windows …
thanks for your help !