Just wondering if this is normal weather or autocallibration kicking in at 06.00 as I have seen this kind of changes at 06.00 before. (but not every morning)
Nope, not auto-calibration - your RH sensor appears to be spot-on! This sort of “spread” between temperature and dew point will happen as the air turns drier. Weather geeks out there, please elaborate!
Actually, it was the temperature itself where I was looking at. It was only slowly changing during the night and then suddenly drops at 06.00. It happened before also at 06.00. I have nothing in my house that turns on or off at that time. It might still be coincidence.
A lesson about weather and sudden temperature drops would be great 
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What time is sunrise in Groningen? ~8:30 local time at this time of year? Looks like overnight temperatures bottoming out with the help of some timely cold air advection. Some other stations around yours also portray this temperature trend. Check the beta map for other nearby station data to compare air temps and winds: https://smartweather.weatherflow.com/map/52.8888/6.7879/9
Typically, the coldest temperatures overnight are reached just before, even minutes after initial sunrise. Solar radiation warms the ground and the ground warms the air directly above it. However, one must also consider warm or cold air advection, which is simply relatively warmer or colder air moving into an area which happens with frontal passages and can occur at any time of day.
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From a meteorological perspective - it is unusual for the low temp. to appear at or close to 9am. Typically the low will vary between 6-7am (summer, winter) when the winds drop off and radiational cooing is at its peak. I’d very much like to see a fair weather morning additional data set to see if this dip reoccurs.
Temperatures were dropping very slowly during the night, as expected. I was just suprised to see the sudden drop at 06.00. It happened a few days earlier as well.
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In my experience, meteorologically speaking, significant drops in temperature occur right before sunrise which typically corresponds to calm winds. When the winds do begin, it is common for the coldest, most dense air along the ground to affect the thermometer which typically shows a sharp, brief decline in temperature. My Air follows this pattern often when winds are calm over night.
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