Thank you so much for all of your answers 
There's a major development, you can see it in my answer to WattsThat.
WattsThat
If the pin 8 voltage matches VCC, there is good chance the output transistor is shorted. If thats the case, I’d remove the 10k and 330 ohm resistors connected to the and test again. With the pin unconnected, it will remain at VCC if the internal output transistors have failed while the program toggles the pin every second.
I made the measurement and saw that the voltage on pin 8 was constant on 4.93V ( = Vcc ) so I followed your message and desoldered both resistors (10 KOhm und 330 Ohm) and now it works and the output of pin 8 changes. So the problem is clearly there somewhere, yet where? I really have no idea why would such a thing happen.
I still added the measurement for the case they might be of interest.
TomGeorge
Can I suggest instead of soldering your controller and SD card boards onto your PCB, you fit them with header pins, that I assume they came supplied with and solder header socket strips to your PCB.
I would be happy to do that but at the end I plan to put the PCB inside a housing which will be glued. I am afraid that if the housing will somehow fall one pin will get disconnected and it will be a headache to bring it back
Can I suggest power switched OFF you measure the resistance across the 10K in this image please?
I did and it was 10K Ohm on both PCBs.
wvmarle
Also I saw on your images you are soldering wires directly onto your PCB. It’s much better to use connectors for that - my favourite is JST-XH, but there are many others, including screw connectors (Phoenix MSTB is a very nice one) if you don’t want to be crimping wires. That makes it much easier to connect and remove wires, and as the connectors are through hole no risk of pulling off a PCB trace.
Ok screw connectors are actually a good solution for the problem I wrote in my answer to TomGeorge. I will definitely do it in the future
PerryBebbington
In reply #21, second image showing the back of the circuit board, it looks to me like all the pads for all the pins down each side of the pro Mini are joined together, shorting out the pins. It might just be the photo or the way I am looking at it. Can you confirm those pins are not all shorted? And apologies if this has been mentioned and I missed it, I’ve not carefully read every reply.
No, they are not shorted, it just looks like that.
6v6gt
Hand drawn schematic. Didn’t you have to produce a schematic to have the board manufactured ?
I did but I had a hand written schematic already so I just uploaded it to the forum. It looks exactly the same though.
No FTDI headers. Isn’t programming difficult ?
I just put the headers from the FTDI in the holes. It is a bit annoying but it works well
Voltage measurement with pin 8 oscillating at 2Hz (1s on / 1s off). I’d have used 5 seconds to allow the meter to stabilise.
I will do that next time, the problem was on the layout though. See in my answer to WattsThat
Thank you all very much, it means a lot for me and I am very grateful for your answers and help