Hello Jacques-Antoine,
1. Use "Lookup" Fn in SAP ByDesign Excel add-in to select values from ByDesign server it will enabled only for certain fields, for the fields which lookup fn is not enable, copy the value and paste the value using "Alt+Ctrl+V (paste only value)" for respective fields.
2. Try to upload single data(row) for testing purpose, which make easy to find out the root cause of error. Once the single data uploaded correctly after correcting the error then upload all the data’s
Thanks & Regards,
Vignesh Ravichandran.