Correct, stack sizes changed from mbed 2 to mbed 5, but the mDot continues to use the standard mbed 5 stack sizes.
I’m not sure why SPI transactions in one thread work. Debugging the lockup with GDB would definitely help you to figure out what’s going on. It’s possible some kind of error condition is being encountered and the system is sitting in a while(1) loop somewhere.
Cheers,
Mike