Good news ! The problem seems to be fixed in my main album
Tested with this config : core V39.1b1, Tiger 7.4.2, no zip file present is the output folder, download activated for scaled-down images, download button shown in the Lighbox, share activated for Threads and BlueSky.
No error is reported in the console on Make and the zip file is generated as expected.
The download controls works as expected in the browser.
Thanks Laza, Jeff and David for your efforts to fix this!