Hello, this error also happened to me but I managed to find a fix.
Here are the steps I did,
I went to the official minecraft website and logged in using my mojang/microsoft account.
Underneath Java Edition I changed my username.
I then went to the minecraft launcher and logged out and back in and it started working again!
Not sure if changing username is required tho
CastAlmania
Hello, this error also happened to me but I managed to find a fix.
Here are the steps I did,
I went to the official minecraft website and logged in using my mojang/microsoft account.
Underneath Java Edition I changed my username.
I then went to the minecraft launcher and logged out and back in and it started working again!
Not sure if changing username is required tho