Maybe something as simple as no contracted maintenance at destination/ no spares?
No RVSM, No RNAV, can't dispatch anymore once on the ground.
There are tonnes of reasons why they might have returned. But to suggest that they can't handfly is absolute BS, they diverted and landed safely by handflying didn't they?
And yes it's a simple reset, but I have seen it fail the reset as well. What makes you think they didn't have a few FO's at it.