New day, new problem !
Some users for an unknown were not able to connect to OWA and Outlook suddenly, I never had time to find why unfortunately…
What I did to correct the issue was simply to modify one digit of the legacyExchangeDN, the last part which is random.
/o=blabla/ou=exchange administrative group (fydibohf24spdlt)/cn=recipients/cn=ef520295b5ab4cc3949fd9e06b7b6285-dbrooks
/o=blabla/ou=exchange administrative group (fydibohf24spdlt)/cn=recipients/cn=ef520295b5ab4cc3949fd9e06b7b6287-dbrooks
Hop this will save time in case it happens to you