AEG Favorit 80860 - error message?

Hi, Wonder if someone can please help me ... I have an AEG Favorit 80860 dishwasher, PNC 91137103200 which has developed a fault.

I switch it on a 96 minute normal/pre-wash programme as usual, it does some pre-wash stuff for a couple of minutes and then when it's meant to be filling up I get a red and beeping error message - 'open the tap'.

The tap is open and water pressure is normal (the second feed from a splitter to the washing machine fills that appliance fine). I have checked all the filters in the manual, which seem clear. I haven't taken the grey filter box attached to the inlet hose apart as I'm not sure that's possible but looking into it it seems clear.

Please could someone advise a newbie?

Many thanks.