Hi Yann,
Is this PSK or dot1x ? Assuming it's not related to DHCP, then we can check the following:
Please collect the following:
show aaa authentication dot1x <profile> | include Delay
show dot1x counter
show auth-tracebuf mac <themac> (when the issue has happened)
note that for the auth-tracebuf you may need to enable per user debugging first to filter the tracebuf so it doesn't roll over so quickly, i.e. logging level debugging user-debug <mac>
regards
-jeff