AEG L75480WD, starts, door lock clicks 3 times, starts to fill then stops

My 5.5 year old AEG L75480WD developed a problem today. I loaded it, switched on, left it at the default 60 degree wash, pressed start, the door clicked 3 times (which is not normal in itself), it started to fill for no more than 5 seconds then it stopped, the timer display dropped to “1” then “0”, the machine beeped and the key and detergent lights flashed.

I realised I was using liquid detergent for the first time in years and a) I’d forgotten to reposition the flap in the detergent compartment and b) I accidentally put it in the fabric softener compartment at first as I was on default ‘this is where liquid goes’ mode. But I washed out the latter, emptied the machine via the drain pump (and cleaned that), then moved the flap and started again, so I trust that’s a red herring.

I tried again a few times, same result. I tried ‘spin’ to drain it, got the same result except the display went almost immediately from 15 mins to 1 min, then “ERR” rather than “0”. Other settings and temperatures did went variously to “0” and “ERR”.

Nothing has changed with the hoses / supply / waste since it was last used a few days ago so I’m guessing the multiple lock clicks mean a door error which needs an engineer, but is there anything else I can try first?