It was not a role but your remark made I tried to remove the group and recreate it and then all is running fine.
Tool (used by our production team ) for migration to fill this dataserver was sybmigrate , which -- at lesat that they said -- take all in charge about logins and users and permissions.
Seems to be an issue during this process.
Anyway thanks for your time , I'm now able to gaive then a process to avoid this issue.
regards
Marc