It's a distorted picture moving arround, sound gets outputed correctly to the speakers as well. Yesterday I used my tester and plugged one end of the Csync out of the CXA1145P chip and the other end to the csync megadrive pinout, it measures connectivity! So there is no hardware between those two elements.
So clearly it is a sync incompatibility thing between the Megadrive with Adapteur RVB cable and the monitor / analogue input board. Perhaps there are some options on the monitor side one can adjust, but I have no experience with this particular monitor. As mentioned, it works fine on a BVM 14M4DE.
The only last ditch thing I can suggest is to wire the sync directly to the monitor going by the video encoder specs:
So basically to the wire you soldered going from pin 11 of the encoder, attach a 470uf capacitor then a 75ohm resistor and then stick the other end in the sync input of the BVM, with all other signals going as usual through the Adapteur RVB cable.
Obviously no guarantees it will work, but the Adapteur RVB cable uses a different sync circuit (schematics below) so this may be worth a try, just to see if that's the cause of the incompatibility.