Hi, Mirko,
Both your measurements and Robert B.'s comment suggests that there is no LO at the ADL5350. What exact was used to pull up the RFoutA to 3.3V supply (I hope it's not the 0 ohm as shown).
As a side note, what impedance did you design the 270 MHz filter for? It does not have a flat response for any source and load impedance I tried, and it's not clear from your schematic what termination resistance was used for the ADF4351. Still, this by itself does not account for the missing LO signal.
Benjamin