I have seen a similar problem with our Vacuflush. It was due to the cassette moving away from the fully inserted position due to the vibration of the pump. It can be caused by a number of things. A similar fault occurs if the “cassette present” sensor has moved and is no longer close enough to the cassette.
Does the pump restart if the cassette is simply pushed hard back into the base unit, this would suggest the above type of fault is present.
In any case I would check the following:-
Is the catch that holds the cassette in place working correctly? Specifically is it present, undamaged, latching properly etc? Also does the fault occur with all the cassettes that you have?
Are both the cassette inlet valve and the mating spout on the base unit clean, undamaged and lubricated? Check the spout seals are not dislodged and the inlet valve on the base unit can be pushed fully in (dislodged seals on the cassette inlet valve can be a problem and cannot be visually inspected without dismantling the cassette, so can go unnoticed)
Is the “cassette present” sensor on the base unit in place and not dislodged and pushed back away from the cassette?
Is the base unit secure, in particular does the vertical section stay upright or is it loose?
Hope this helps.