2 hints seen in the forum:
No special characters in PSK: http://forum.mikrotik.com/t/rb951g-2hnd-unicast-key-exchange-timeout/104896/1
Setting Advanced ‘distance’ to “indoors” instead of “dynamic” may stabilise the timing for key exchange. One can even set a value : http://forum.mikrotik.com/t/wireless-problem-android/148451/21