Thanks for the pictures.
From the cables on top of each other it is hard to be certain about the wiring. The Xsens should get power (11.7V) on the yellow and yellow-green xsens wires. Then the Xsens Rx (brown?) and should be connected to the RS232-output of the max232. The input for this driver should be the umarin-Uart0-TX pin. When you start the umarin you should see a few 0-3.3V pulses on the uart0-tx and a few -5V -> 5V pulses on the xsens-rx wire trying to send config data to the XSens. Then the xsens-tx (white?) should go to the rs232-side-receiver input. The 3.3V output of this receiver should go to umarin-uart0-rx. When all goes well you have -5 to 5V pulses on the white wire which become 3.3 - 0volt pulses on the umarin-rx. If you can verify all these, then it must be a software issue. But given the amount of cables, connectors and own converter boards it would be good to verify some things with e.g. an oscilloscope.
