damned, it seems to be a clock init problem. I’ve stepped thru the SetSysClock() and see that the HSE is wrong, it does not fit for my custom target. HSE fails and the fallback HSI is used.
In my testcodes I have printf for Mbed Version and a hello message, usually this fails already when the clock is not configured correctly.
edit:
but the problem is still there, even with fixed HSE clock 
There should be 8 data + 1 parity bit, but the analyzer is telling me there are 8 data bits.