There was a 60 Hz and 120 Hz oscillation on the green PDH photo diode output. After a long search, I could identify that
the source was a broken BNC cable which was connected to the photo diode. I exchanged that BNC cable and the 60 Hz
and 120 Hz are gone :-)
With the new cable the PD output was less noisy so that it was easier to achieve a better alignment of the light to the cavity.
The reflected power could be reduced from 40% to 30%. For perfect alignment the reflected power would be 20%.