This could be caused by a blockage in the macerator mechanism, water tank or outflow pipe. Also a failing microswitch on the float valve would cause similar problems.
Switch off at the mains and open up the unit to see if you can locate one of the above issues. I can assure you, it won't be pretty in there and you'll need decent gloves and a strong stomach!
Below you will find a useful link on how to sort problems with Saniflow units:
Poor installation of the outflow pipework can also cause problems, make sure the pipe exits at 45 degrees - the installation instructions will show what I mean by this.