Can't whitelist a player

Hello,
so I have this little server for family and friends and I use whitelist. Now I just can’t add a player named Kacenka in there! I add it like any other player
whitelist add Kacenka
But the game still kicks me out and server says

[20:18:52 INFO]: Disconnecting [email protected][id=fad0079a-ed15-3d95-989a-93fd8e987eb7,name=Kacenka,properties={textures=[com.mojang.aut
[email protected]]},legacy=false] (/myIP): You are not whitelisted on this server!
I then tried
whitelist add fad0079a-ed15-3d95-989a-93fd8e987eb7
but still nothing! That nickname can only connect when whitelist is off. What is the problem?

so I have manually edited whitelist.json and modified UUID. How does the command whitelist add playername know it’s UUID anyway if it’s not online??

Well it queries mojang’s backend to find out what the uuid for that username is.