Server corrosion, air conditioning and climate control
The way an air conditioner dehumidifies is (in brief) the high-pressure refrigerant is forced through a small opening, leading to lower pressure on the other side of the opening. That decompression lowers its temperature; that low-pressure side starts near your evaporator (indoor) coils. Air is blown through the evaporator coils, cooling it. Moisture from the air condenses on the evaporator coils, just like it does the side of a cold drink. That water then drips off the coils, to a tray under them, and then out of the unit's drain (or on some units, into a reservoir which is then pumped out when its full enough).
The moisture has to come from somewhere. Even if somehow the AC evaporated all the condensate, you'd get the same absolute humidity (dew point) you started with.
So there are a couple of likely places it can come from:
Outside air is infiltrating. Looking at the current conditions in Groningen, it appears the outside dew point is 16°C. That is awfully close to the dew point you measured with the A/C off.
How good is the air mixing in the room? Is it possible you're cooling just a corner, and the rapid humidity increase after the AC turns off is just the air mixing? (E.g., is your AC intake next to the output?)
Your drain line could be broken somewhere, and is dumping the water inside your server room, and it's then evaporating back in to the air. Alternatively, the water could be staying in the AC unit (and somehow not building up enough to spill out on the floor) Verify the drain line by inspecting the entire path and confirming water is flowing out.
If you have humidification, it's possible it's running out of control. E.g., it's humidistat may be broken.
You didn't say what test equipment you used to produce that graph, but if it's not accurate...
Also: Your AC appears to cool the room really quickly. This reduces its ability to dehumidify—it's quite possible your AC is simply too big (powerful) for the room.
Your statistic don't lie, get a new A/C.
In freezing mode the A/C kill the humidity, its just not freezing. Your stat show the inverse, its illogical, please change the unit, or buy a deshumidifer in the short term
There were some good answers, but the details deserve a separate answer, I think.
The comments about the AC unit being too large seemed to be closest: the condensated water never reached sufficient quantities to be drained out because the thermostat cycle was so short, so it got evaporated back. I was advised to turn the temperature up to 24 C, and set the fan to the lowest speed (it was set at highest, arbitrarily). It seems to have made a difference:
Not only is the humidity lower, but the spike that normally coincided with rise in temperature (when the AC was in the off-state in the cycle) is gone; There is now a corresponding increase in temperature vs decrease in relative humidity, so probably the absolute humidity stays the same.
Also, the dew point graph is a lot flatter.
Your rel. humidity is quite huge, standart for datacenters is 50% +-15%, you're reaching 80%. Check your AC settings if there are no option for increase humidity enabled, this just looks like not so healthy place for your servers.
Check if there are not any wet places on the walls, ceiling, floor (look under panels if you have floor with two layers). Are there any windows or holes in walls and ceiling (like for cables and pipes)?