First of all: thank you all very much to have a look into this issue (if its really one)!
@vicentiu, its a DSOX1102A (https://literature.cdn.keysight.com/litweb/pdf/5992-3484EN.pdf?id=3019454) with about 1 MΩ ± 2% / 16 pF ±3 pF. The scope probes I have access to have the following specs:
Keysight N2140A (https://literature.cdn.keysight.com/litweb/pdf/N2140-92002.pdf?id=2803394) with 15pf - 30pf comp range - tested, undercompensated.
Probe Master 4902-2 (https://probemaster.com/4900-oscilloscope-probe-basic-kit-150-300-mhz/) with ~10pF - 50pF comp range (according to the page)