I do realise you solved it.
GeraldR63:
To make the Sketch work and upload it it took a lot of "USB plugged out USB plugged in" and reset.
But since added the Bihor no serial seen anymore.
The first test to do is disconnect everything and try again.
GeraldR63:
I read that it is possible that COM will not work anymore if the loop do not have delays?
There is one in the sketch (delay(100)).
Don't believe everything that you read on the web; there is too much rubbish. Find at least a second opinion that confirms that B.S..
GeraldR63:
How to really reset the UNO? Not restart with custom sketch to get the UNO back I removed yesterday from the package?A RESET button should RESET the WHOLE system and make everything work the way it came from factory and not restart system with custom software. In my case, I guess, this would avoid a lot of trouble.
Hmm, a reset button on your PC also does not do that 
GeraldR63:
The UNO OS should also have enough CPU time to serve the native UNO functions like COM handling. I guess that this is an issue but I will try in the next step to remove all of my work from GND.
There is no OS. When you press the reset button, the boot loader runs first; your code does not run. The boot loader looks for a little while 'at the com port' if you want to do an upload and after that it hands control over to your code.
GeraldR63:
Nice device but for kids too much trouble. Plug and Play would be nice. However when it works it's really great.
It is plug-and -play as you have found out 8)