Opening serial port causes kernal panic

I'm running Armbian_5.20_Micro_Debian_jessie_3.4.112 on an Olimex A20 micro board.

I find that opening serial port device `/dev/ttyS2` causes the system to crash. The board heartbeat LED either stops or blinks rapidly. The board itself is not pingable.

I can reproduce this issue with a simple from the shell using: 

cat /dev/ttyS2

 or from c code using:

fd = open("/dev/ttyS2", O_RDWR | O_NOCTTY | O_NDELAY);

`/dev/ttyS2` is linked to UART6. In the fex file, I have configured it like so:

[uart_para6]
uart_used = 1
uart_port = 6
uart_type = 2
uart_tx = port:PI12<3><default><default><default>
uart_rx = port:PI13<3><default><default><default>

Note that the UART6 is on MUX3 as it should be according to sunxi's pio documentation for A20.

Any ideas?