i have ban a player yesterday with rustadmin. i decided , after reflexion, to unban him. i did it also from rustadmin, and he could come back. but today, he is banned again from my server without any actions of my part and  i can not see it in the "bans" of rustadmin.
any clue ?

Have you tried stopping your server, going into your bans.cfg and removing the ban from there and restarting? 

Bans and ban sharing in RustAdmin are bugged since the last month's update. Try clicking the REFRESH button on the bans page. Also, you need to unshare the ban BEFORE unbanning someone, or else they will be unbanned from your server but will show as banned on RustAdmin's database.

fixed.
i refresh it, the name appears 1 more time. i unban it.

Did you unshare the ban first? If you haven't, the player will be unbanned from your server but still on RustAdmin's ban database.