Lamona 8631 Regeneration tank overflow (leak)?

Lamona 8631: Has been working OK for over 3 years but today, water pouring from a square hole at the top of the regeneration tank (behind LHS left side panel) when the dishwasher fills with water. When filling stops, the level in the regeneration tank lowers (water stops flowing out) and the machine works very well. I am very puzzled! HELP PLEASE. P.S. If I put my thumb over the hole to stop the water - no adverse effects as far as I can see)

Tim Rollinson
July 2017