"Be Sure you are online"

My friend experienced en arror today (21.01.2021) that means "Login flailed. Please be sure that you are online." or something like that.

What we tried already:

-End Win10 Firewall (he doesnt have an Antivirus installed)

-Java (and the MinecraftLauncher.exe) are now checked under "Private" in the Firewall

-Login on an other PC (worked!)

-No programms like MCLeaks are activated

-Reinstalled the game several times and deleted the .minecraft folder

-logging in on minecraft.net (didnt work on his PC)