Stupid bug finally found: the stack of the idle task was not big enough to accomodate the UART interrupt running free at higher level.
The result was the corruption of kernel structures.
Will for sure implement a passive stack overflow check, running in the idle task, that verifies the integrity of the stacks of all running tasks.
issue solved for the UART when less then 3/2 chars are sent (lost interrupt). Solved by waiting for the "done all transmitted" interrupt. Still not perfect if an int should get lost.
Should not be an issue for the MX7xx erires with 8 deep buffer and asserted ints (not simply cleared)
UART tested successfully up to 115200 baud. Could probably tx even faster.