Thank you all for the fast answers.
The drop means lost then, and all axis. Sometimes just A3-A6 but most common all axis.
Yes, the earth bond is correct even we called KUKA GMBH and they came to approved it.
The history is simple 15 years worked in Sindelfingen, 6 In Kecskemét and now this problem occures. Till this time (it means 3 months ago) it worked well.
Got no other message, just the mastering lose, and the motion enable stop message.
This robot is a handling robot, and not even overloaded. 3 welders, and a glueing station next to this robot.
Not yet gotten any common factor, once even came this problem, when we got back from lunch pause, and the robot lose mastering in his standing state.
Thanks for the encoder check, we will try that. maybe the other 3 welders do some problem.
We will start to separate this robot from the welders, or maybe we change the robotbox with another from another line. Who knows, maybe this problem will disappear on that line, and the place got some kind of "static disturbance".