I had this same problem with a Stanley door opener. In my case, it was a matter of the power cord was not completely pushed in for good connection in the receptacle, so the voltage drop when the motor kicked in caused the contactor to drop out.
Have you ever adjusted the safety tensioners? Have one of them come lose? Usually, a tensioner will cause door to reverse, but if it kicks too soon, the contactor will drop out and not allow reverser function. Before looking at the complicated stuff, disconnect the operating mechanism and see if the door moves freely. (Whoops, see you tried that) How many times I went to repair a garage door opener and found a lose bracket on the door to be the problem I can't count..Good luck, Kizmet