Linear Actuator of the boom of 42006 jams I just built a 42006 excavator, and the Linear Actuator jams such that it can't be rotated. I think that the overly sharp angle of the u-joint at the top of the boom is the reason.

Is there a workaround?
First off, you should check the instructions to see if you made a mistake before going into some long repair method. Even if you find no mistake, what I would do is disassemble the entire crane assembly and then reassemble it; this would fix problems you may not have noticed. Another thing you could do is remove the actuator and fully extend and retract it a couple of times. If you feel any extra friction while doing this, than the actuator might need lubrication.