Bosch SMS 5012 wash cycle ends after just a few seconds?

When this first happened I started by investigating the under-sink plumbing.

There was a non-return valve where the washer drain pipe connected with the sink waste pipe. This was badly contaminated/blocked (we'd had this appliance for many years, so I shouln't have been so surprised!). I cleaned this up and everything worked. Brilliant! Cycle failure was clearly due in an inability to drain residual water prior to recharging for the next wash. Or so I thought.

But success was short lived, the same problem has recurred, and no matter how many times I remove, clean or re-orientate the non-return valve, I can't get the machine to progress after the first few seconds.

Except one one occasion, having accepting that I'd be doing my washing up the old fashioned way, the machine started spontaneously (i'd left it mid-cycle). What excitement! I started using it again. But after just a few washes, it's gone back to its old unhelpful ways.

Why?

I think it's got something to do with the drain cycle, but how can I make it more cooperative?