Hi Sébastien,
It reminds me some tests I did last year, while starting to work on HANA... From what I discovered, DI-API is updating all documents at once when you try to modify any one of them.
So if you try to modify one then a second, the modification is already done.
Besides this, the named index effectivelly doesn't exist... Perhaps an inconsistency in the database you're using? Did you tried to do the update on a clean (just brand new) database?
Is the DI-API you are referencing in your program consistent with the installed version of B1?
Regards,
Eric