Hello again,
first of all thank you so much for the answers, it helps me narrow down the problem!
I've monitored the system for the past 4 days again, this time with the default reference Vcc.
It looked fine the first 3 days but on the 4th there have been massive fluctuation again. Again, nobody touched the system in any way.
I might have an idea where the problem is coming from:
I was there this morning when the system was still inacurate and measured Vcc, it was 5,08V at that time. Some hours later it dropped to 5.02V. So it makes sense that the AnalogRead differs.
That's why i wanted to use the LM4040, but unfortunately i purchased the 5V-Version.
Because there is hardly an overhead driving a 5V-reference from Vcc, the voltage dropped as mentioned before to 4,84.
When Vcc went up above 5V to 5,08V, i think the LM4040's reference voltage has also risen, since it is supposed to provide 5V.
I can't confirm this yet but could this be the case?
I could try getting a 4,096V reference, but that means that i can only measure voltages up to 4,096V? Or is there another way to get a stable reference while being able to use the "full range" of the arduinos 5V?
Thanks in advance.