jhautz, I've never done what you have on a Lehner before, but I do know that it takes heat to desolder; and if your motor got hot enough to desolder the connections that you made, the desoldered connections would be the least of your problems. Obviously this is not what happened.

In what manner did you do the Y connection? Without having seen it, all that I can suppose is that in your effort not to overheat the little 3mm connectors on the motor when soldering, perhaps you made a series of cold solder joints that literally fell off. Or perhaps you used some solder that had a really low melting point. I know that I have some silver bearing solder paste that melts with the heat of a match.