This a fun one - the Optical Water Indicator isn't working. Not surprising really, as it isn't fitted to your machine.
What it will mean in this instance is that the Water Indicator (WI) isn't working. Remove the filters and look down into the sump; at the 3 o'clock position you will see a black grid. This is the WI, and it will probably have become filled with congealed fat. You can try attacking it with a toothbrush and very hot water - if that doesn't do the trick, it will have to be removed for better access. It's a twist fit from underneath - power off first and beware of edges sharp enough to shave with.
Also possible that the microswitch has failed in some way. If the worst comes to the worst, it's this item HERE