thanks for all the replies!
my issue is now solved:
the charging port of the phone has different connections within (like - for high voltage charge, low voltage charge, data transfer etc).
apparently, some of those got damaged.
I took the phone to a very good technician, that replaced the charging port (cost ~ 30$).
he did it without removing the screen - this is important - because the screen is glued to the body, and may get cracked. he opened it from the rear side.
now all works fine - the charging and the PC data transfer.
I hope I helped
As a manager of a cell phone repair store, I am curious how the technician replaced your Galaxy S6 Charge port without lifting the screen. The reason I ask is that the samsung charge port also contains the soft touch pads for the menu and back soft touch keys on the opposite sides of your home button. Every Galaxy S6 charge port I have in my inventory contains these crucial components. Did this tech solder the usb port on to your device?