I checked the firmware, only after 10 subsequent failed transmissions a wrong temp. (here: 40°) can occur

I am quite sure that there are interference problems. Please test without extension.
@Kartyse: Show us a picture where we can see the temp probe and the cable to it


PS: please let us not mix here too many topics. I see no relation between the level inputs (are the really errors or just correct readings?) and the cond. probe (why do you have 38mS? please check if this value is shown in "normal operation)
if there are issues with other sensors than temp please open new threads