Hi Yves,
Z-stack is using both IDATA STACK and XDATA STACK.
Please when you debug monitor the real stack usage --- View --> stacks and verify one of the two stacks are really full.
However, I am not sure you're really having a stack overflow - I think this is just a false alarm. If you're using power management, the debugger may indicate false stack overflow messages when polling the status of the part, if the part is in between a transition from active mode to one of the power down mode.
If you confirm that really there's no stack overflow with the test above, just ignore the message (to do this workspace click right Debugger ---> Texas Instruments --> Target and disable the 'Enable stack overflow warning'
Thanks,
TheDarkSide