Some more information re the two different errors...

The sequence of events seems to be:
  1. Data\Metadata adapter service restarts
  2. RobustNetworkedMetadataProvider fails, recovers, and requests a snapshot of data from adapter
  3. My adapter takes ca.10sec to start up as it loads a lot of historic data, so in the meantime LS Server is trying to request data for items that my adapter knows nothing about yet, and throws "Generic Exception" errors

Then, any time after this happens, if a client subscribes to items via the webpage, the "Unexpected updates for item..." is logged and a full snapshot is not sent (in the LS log file I can see a new session being attached and controlled), but updates do come through.

If I then refresh the page a full snapshot is sent as expected.