Thanks, it worked!
During the upgrade, it froze again on the database, more precisely on the search_index table.
I made the requested queries manually, as recommended, then I restarted the upgrade, but it started again as before, going back to the beginning of the step... So I tried the automatic database correction, but strangely enough, it worked!
However, there have been no changes to the default theme since version 4.7.4?
Edit : And I have one last problem, related to this one: despite the successful upgrade, it left me the error message in the admin panel about the database having a problem, with the display of the queries to do to fix the problem, as before the upgrade...