Siemens dishwasher SE30290SK not filling

[JM2840]

Dishwasher Siemens SE30290SK
Owned about 3.5 years (second hand). Not moved or otherwise messed with in that time.
About one month ago the machine started to wash poorly: seemed to be an absence of filling, because if we added a couple of litres of water via the door then everything was OK. Often just adding water at the initial rinse is enough, and the cycle seems to complete normally, but sometimes extra water needs to be added later too. The problem is intermittent, but worsening.

What I have done so far:
Checked and cleaned the 2 filters in the aquastop (where it attaches).
Taken off the left side, and confirmed there is no water in the base, that the float is not activated (stuck up) and that the flood microswitch works but is not activated.
Found the connectors for the aquastop at the back of the machine, the resistance across the two is about 2000 ohms.

The water pipe from the aquastop runs direct into the filling tank on the side, no other valves/switches on the way.

In the last 3 days, the tablet dispenser lid has started opening after a couple of minutes (ie during the initial rinse instead of during the proper wash). When I shut the lid, it seems to fit securely and cannot be opened just by shaking/banging/rattling although if I lift it slightly it will pop open.

So where do I look now? Have I got 2 separate problems or one problem (which means likely to be the PCB right?).
Any and all help appreciated.

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