Teams Connection error 4c7

recently I had a connection error on Teams for a user not yet migrated to 365.
user can't connect to portal.office.com nor to teams.

Root cause was that on premise the mailbox was shared since it was used both by a manager and a secretary.

when mailbox has been recovered to normal login to office immediately worked.

Desktop Teams application was still fiailing. Issue was the SSO, so modified that file :

%AppData%\Roaming\Microsoft\Teams\settings.json

"enableSso":true/false