Sony HAP-Z1ES + MacBookPro

Love my HAP-Z1ES.  Used it to feed analog into my pre-amp; now using its digital USB output to feed my Devialet 200.  Truly happy with it.
Alas: this week, with my latest MacBookPro software upgrade (MacOS 11.3.1, an update to Big Sur) I can no longer transfer files (using the HAP Music Transfer app) from the laptop to the Sony.  I get error messages, either "Some files are failed in transferring. Please check the transferred file list." or "Transfer cannot start because the device has not been ready for it."  (I've never gotten either of these before.) 

I pinged Sony via Twitter but don't really expect an answer.

Everything else (playing existing files; using the HAP app on my phone as a remote) works fine.

Thanks in advance for any Audiogoner who has advice, diagnostics, workaround.