We solved the problem!
We panicked a little, but fortunately we could solve the problem
We discussed the problem with our friends, trying to figure out what the problem could be. We even visited a local electric gadget store to rule out possible problems. And although no one pointed to the root cause of the problem they started a flow of events which led to the solution.
One of the suggestions we got was to check the resistor, to make sure it didn't have a short circuit. We checked this and the resistor seemed okay. But to make sure we wanted to connect something instead of the resistor to check if the problem still persisted.
And during the wiring of the main board we realized the connect point of the main power source is sloppy, like the soldering has lost the connection. It was a tiny cluck, but it was there. And after we re-soldered it, our little electronic problem has been disappeared!
Unfortunately it seems the filamentThis is the plastic which is used as a source for the printRead more pushing still has some problems, even with the gold filament, because the first test print has stuck again. But this time the motor seems to be strong enough - our suspected culprit is the hobbed bar, because we saw it has broken teeth. Maybe that is the cause it slips so easily on the filament, so we try to get a new one.