Some precisions: when the upload fails, it goes in the history of the phone app as failed.
From there, if I click the failed files, the uploads restart and succeed even when I click on several files in the same time…
I can see no error in the server logs. I think the problem is in the client side.
Thank you for the answer, in the gallery app you can select multiple files and use the share tool, but it bugs (on Lineage we retried on android and it seems to work).
I found a workaround: auto share the phone picture folder to a Nextcloud folder so that every take picture is directly transmitted and I sort later… but it is a bit data intensive.