Thanks. I managed to fix this puppy. You have just to pull the back cover firmly.
The problem was not a solder joint as I first thought (I reflowed the joints anyway), but the inner diameter of the sockets for the remote button were (maybe) out of tolerance or maybe the plugs from the test lead are out of tolerance. The plastic housing was manufactured so precisely that the plugs were sometimes not touching the socket walls because they sat exactly concentrically.
After some fiddling with this issue I decided to wrap the back cover in aluminum foil, made holes at the proper place and by using hot air I made the plastic where the sockets were mounted kneadable and pressed the two sockets a little bit together. After cooling down I reassembled this puppy and now: It works! No loose contact anymore!
