Thanks to the UK Trilobite specialist I now have a brand new gearbox/motor unit for my ZA1 (for a very reasonable price) and it is moving again
In fact I was able to get quite a few good long cleaning cycles around my home and was very impressed (shocked!) by the amount of dirt it found
But it always failed with the fan motor doing strange things- speeding up and slowing down until eventually failing with an "error 4- service" message. Several times I was able to get it going again by lifting the bumper and re-seating the fan motor connector, but eventually it failed completely and the fan no longer spins at all.
I rang the specialist and he told me that it was common for the fan connector to get corroded and cause such an issue. He also confirmed that the error 4 is indeed a fan error- specifically that "a signal was sent to the fan but it did not respond" (paraphrasing). He also said that he had never seen a fan fail- it is a very high quality Papst unit.
So I took the fan out and tried everything I could to revive it... but it is dead as a dodo
I believe that it is the fan as I get zero continuity between the 2 thicker power wires. It's not the connector. It could still be something failed on the motherboard but I would expect to get some resistance reading on those wires but nothing. He's sending me a replacement fan to try so fingers crossed.
Another thing I noticed with some concern was that the actual fan blade casting was quite badly cracked like it could fly apart at any moment