solved. The problems i had has been level shifter dependend. I used another one and UART started to work. Another trap has been a misunderstanding in PIN numbering and PIN layout. When i just looked at WiPy pinout without paying attention to expansion board i noticed, that i used the wrong PINs. Now i count from left side, starting with 0 and figured out which PINs i do have to use. Now it works like a charm. Thanks for all your help !