It's an effort to decipher a circuit from the wiring, especially as the symbols for BC107 and BD135 don't match the physical appearance of these components.
It will be much easier for us when you present your circuit as a
schematic diagram.
Also, in case you can't draw a schematic as requested, at least use red for positive supply wires, blue for negative supply wires (gnd), otehr colors for signals. Also label any signal/wire that you reference in your text. It is much easier for us to follw e.g. a signal named "output" than to fin a signal that "connects to the collector of the second NPN transistor" - just an example).
For the time being: check the correct placement and assignment of the transistors and their pins (E, B, C).
Now I measured again the voltage at the end of the blue lines.
Please be more specific. Ideally mark the measuring points in the diagram with lables and reference these lables in your text. The blue wire (in fact any wire) has 2 ends. Which one do you mean?
when Arduino's pins set to low, 4 of them show 6.8V, one shows 12.2V.
12 V on an Arduino output? Your poor microcontroller sure is fried. Check your zener diode, is it correctly oriented (cathode to +12 V)? Check your ground connections.