I haven't had that model, but on other softeners, it's happened when the multiport valve that opens and shuts various waterways has worn ("O" rings) or jammed (scale) and is letting water flow through slightly when it should be shut. IIRC it takes about ten years. On one occasion I exchanged it for a reconditioned machine at a reasonable price.
On mine, there has been a service/overhaul kit to replace the moving parts in the valve assembly. Mine had a Fleck mechanism which seems to be very common in numerous makes and countries.
It might be possible to take the valve head off and dismantle it on the kitchen table yourself, provided you can identify the mechanism and order the kit, but I found it easier to get my local Softener company to do it.
If it helps at all, my Fleck valves (there are several models) look something like
and the kit of piston, seals etc looks something like
Of course, yours might be completely different. I think the kit was around £60 plus a bit over an hours time for the installer. Cheaper than buying a new one. The mechanism is actually fairly simple.
Some cheaper brands appear to be irrepairable.
There are some electronic control heads that I have avoided because I can't see how they work.