Whitelist not working properly

Hi, every time when I and my friend try to play with an Aternos server, I always add him to the whitelist, both by the console command /whitelist add [Username] and by the classic Aternos way, but the game keeps kicking him telling him that he's not whitelisted while I can normally join the server and play in it. I tried reloading the whitelist by the console command /whitelist reload and trying to add him again but still doesn't work. How can I fix this error?

Quote from Roman

Stop the server, remove all players from the whitelist and add the players again.

then start the server

Thank you, it works!