It does indeed solve the problem!
Thanks @pythoncoder (and @Damien) for not giving up on this one.
Looking at the board diagram, the information that I2C1 pins are shared with UART1 is indeed there - but, at least for me, it was not clear that you are prone to problems if you use the (internal) Accel() and UART1 simultaneously. A side note / warning on the Accel() class documentation would be useful.
So, case solved.

Thanks guys!!!