Daniel,
Thanks!
In fact it was something similar to that which seems to be causing it.
(Though that could be incidental): I have been re-importing all my box sets properly split per these guidelines. Each time the (‘new’) files are imported, I get the stall 
.
I may be misremembering, but I don’t remember seeing it before I switched to Nucleus recently - although that seems unlikely!
I’m sure I saw another progress of ‘x/y’ just under the Settings > Library > ‘Clean up Library’ button before. Now I don’t. Maybe that’s with the advent of version 2’s split of server and client?