AEG Oko Favorit 64800. Not enough internal pressure to make spray arms work?

Hello. I've got a rarely-used UK-spec AEG slimline dishwasher, Oko-Favorit 64800 model. It stopped pushing water through the spray arms, and displays an AL5 code.

I've checked the system for blockages, and water is flowing throughout the system. The filters are clear (including the one at the tap-end of the inlet hose). As it's a lightly used machine, internally it is pretty much spotless.

The dishwasher pumps out all the water at the start of a programme (as it should), and then fills back up (as it should). Then the circulation pump switches on. I can see the impeller spinning in the circulation pump, but it has a rattling sound. After a few minutes, the AL5 code appears.

Water is rising up the pipe that goes up the side of the dishwasher to feed the upper spray arm, but only goes up a short way. It's almost as if there is not enough internal pressure for it to work properly.

Has anyone got any idea? I've tried everything I can think of, but I still have the issue. I've also tried to find a service manual or even schematics to show how it works, but to no avail. If anyone knows where I can find this, I would appreciate it.

Many thanks in anticipation.