My question then - in your language, are the two adequately distinguished? I'll bet they are.
Got to answer that of course.
Soldering translates to Dutch as "solderen"
Welding translates as "lassen".
But that last word, lassen, can be different things too.
They all have to do with bonding or connecting however.
First thing that comes to mind, is twisting installation wires together using a plastic cover, like these:
Is called lassen in Dutch, but it isn't welding.
I understand you were trying to convey and explain the difference.
The "no idea" part was what triggered me to reply to it.
I don't believe that this is about someone not knowing what he was talking about, just someone who has no idea about the differences in other languages.
Back on topic:
Today i can see the picture.
The wires are the well known jumper wires, they are also well known to have bad connections now and then.
Those wires are not meant to be soldered, and i'd not be surprised if that would be the cause of your problems.
Also crossing wires is more likely to happen if you are using the same colors and it seems to me that you did do that.
I'd use as much different colors as possible, but keep red for power supply and black for GND.