I'm sorry, but I don't. cn=manager,c=fr was already the rootdn of config database (as well as rootdn of c=fr database) before doing the test, with no rootpw in the config database. My first test consisted in modifying both olcAccess and olcRootDn for all databases (if present) without changing any value (replacing the existing values with the same ones). Then I made another test, with olcAccess modifications only (on both frontend and config databases), and the problem still appears. It only appears if I modify olcAccess of both databases, and in this order: frontend then config.

You do, since you modify the configuration using the cn=config rootdn in order to modify itself, and set it to cn=manager,c=fr. So in the first place you had a rootdn in cn=config, with a rootpw. And the modification you posted did not remove the rootpw.