Hi Alex !
I think i found two errors.
On the part list for the boom only 4 parts Nr.:61904 ( Linear Actuator Holder ), needed 5, forgotten by step 43 in the boom instruction.
Missing 2x Part Nr.:6542b ( Technic Gear 16 Tooth with Clutch ), 1x Part Nr.: 6539 ( Technic Driving Ring ), 1x Part Nr.: 32187 ( Technic Driving Ring Extension ), to connect the steering mechanismen from superstructure to the Base.
Hans-Peter