It may be related to the timeout in reverse proxy…
I set the timeout to 300s = 5min :
And, then import again my big JSON file with 776 entries.
And now, no errors ! In the Admin panel, I can see this for my user :
But my method to select all entries and try to delete them in order to see how many entries I had… it doesn’t work…
Look what I have after the successful import :
So, the reliable way to see how many entries I’ve got is to go on the admin panel 
I hope this would help someone someday.
Bye