Tested with the "0 + ++" sequence (shortest exposure first, exposure time increasing on subsequent frames):
Advanced bracketing seems to fail on the frames requiring an exposure longer than 30 seconds, but only if the shooting is initiated from live view. When the camera attempts to begin exposing for such a frame, two things can be seen on the LV information overlay before the sequence silently terminates:
1) the mode display briefly changes to "B". 2) the shutter speed display briefly changes to "BULB".
If you exit live view before initiating the shooting sequence, then the exposures longer than 30s will work fine.