Hi everybody,
latest news from my issue with the touchscreen: It is NOT A SOFTWARE ISSUE!!!
This post is a bit long and a kind of a report – be patient and read it 
.
It is somehow a hardware issue which I can’t explain at the moment. As well it is to mention that I have found the solution for me a little bit coincidently – but still have some question marks in my head!
What I have done since my last post:
I got a multimeter from a friend and measured the touchscreen support directly at the 5V pin to the GND of amp2. (And promptly I destroyed my new – even not one-month old pi 3b+ while measuring because I slipped with the tip of the contact and build a bridge between the 5V and 3V pin, bloody hell 
and all this because a very trivial issue but luckily only the pi is destroyed other components are still fine).
Well before that mishap I measured about 5.06V at the pin. Then I had to change the pi and installed my pi 3b and measured again – very carefully – and measured again 5.06V, so power supply at the pin is ok. Next step – I connected the jumper cable for the touchscreen support to the 5V and GND pins on amp2 and measured at the end of the cables which will be connected to the display board – here as well 5.06V, so still enough power there. During all this the screen faced down to the table to get good access to the pins. Then I took all the construct with display, pi and amp2 in my hand – this is the coincidently moment – and tried out to navigate with the touch function and Look! It works perfect even the amp2 was attached! Then I played some music and it worked as well and I could even use the touch function at the same time while the music was playing without any problems.
Maybe one more thing that everybody should know – I assembled some brackets to the chassis of the display at the back to put it straight upward for a comfort use and look. But unfortunately, the free space between the construct and table is a little bit small and at the same time the power supply connector of amp2 faces towards the table and the barrel-connector of the power supply is a bit long, so when it is connected to amp2 the space is even smaller – too small to put it on the bracket’s feet. Therefore, I positioned the whole construct somehow in the box of the display with the foam inside for having as less bending on the supply cable as possible and this is one of the positions when the touch doesn’t work.
Nevertheless, I measured the 5V pin in this position as well and power supply was ok – 5.06V but touch function poor, so it can’t be a power supply issue. With this finding I took the construct in the hand again, hold it at the bracket and bended the power supply in many directions – small bends, big bends of the cable, jiggled and shifted directly the barrel-connector – and tried at the same time to navigate with the touch and there was no problem with the touch function, it worked perfectly. Then I tried jacking up the brackets at its feet to get more space between the construct and the table, so that the cable can hang little bit more free and straight. Subsequently I put the construction on the table and not in the box again. Result – bad touch reaction. In this position I measured the power again, still 5.06V. …???.. What is the problem? I don’t know! Doesn’t matter in which position and constellation only the cables and power supply are connected in a right way and firmly and there is no other contact of the components with anything else so that we can exclude any possible short circuits or something similar.
Someone told me to shield the ribbon cable between screen board and pi by wrapping the ribbon cable with some aluminium foil, which I did already before (and which didn’t help). Based on that ideas I put the supply cable and loudspeaker cable and with other parts and cables closer together to see if there are any EMC issues – still good touch reaction when I hold it in my hand, so that can’t be the problem either!
When there is the problem with the bad or no touch reaction – sometimes while trying the mouse arrow appears in the upper left corner of the screen but doesn’t move. So, I guess sometimes the touch reaction is been gathered by the system but not used.
But here is another phenomenon – physically I keep the construct in that position in my hand in which it works perfectly as described above and don’t change. Then I change only the config.txt to rotate the display 180° and reboot. After reboot I see everything upside-down in the display and try to navigate with touch – nothing happens, so no touch function even the supply cable is in that position in that it worked before rotating the display via change of config!
So, hopefully now everybody can understand why I still have question marks in my head and can’t explain the behaviour! But fortunately, and/or hopefully I found now my solution – after I will be done all my implementation – I would install everything in that way as I would hold it in my hand and hope that it works!
But this is pitiful and pelting and keeps still some doubts that it will work in the end when everything is ready, and I put it in a nice box. I would like to exclude the doubts and be sure that it will work fine when I am ready with everything!
So, if anyone have an idea what the problem might cause – go ahead and tell me!
Best regards
PS: If someone wants, I could make some Fotos of the whole construct and post it here!