Hi @antonin_d,
Kudos on your recent then, because it's working. For reference to anyone facing the same situation, I would have liked to provide any useful information, but after upgrading there were no error messagew and the data is now on the wikibase. I'll keep testing on other spreadsheets.
The only thing that I can mention is the following. Our upload consisted in the addition of a 560-line spreadsheet. These were 560 statements to be added to 10 different items. While the wikibase now seems to show all relevant statements for all 10 items, the "Wikibase editing results" panels that appears in the left-hand-side column displays "no edit 550, successful edit 10".
I remember reading that edits were grouped (unlike in QuickStatements), so, in a way, 10 edits for 10 items is correct. However, saying that in 550 instances there were no edits is incorrect or misleading. I hope this helps.
Thanks again for the quick help and I'll try and flag anything relevant going forward.