I'm an employee of internet service provider.
So, I am trying to solve one customer's problem: he cant pass RADIUS authentication with correct username and password. According to the client, he did not changed his device properties since all things where right.
You need to talk with your backend developer. It's a backend side error message.
In code, there are lines that are throwing that error. According to the error message, It's possible to account is deleted and that's why error message 1 - Invalid account, when email/username exists in DB backend must throw an error like your username or password is incorrect.
But it depends on how your program is handled in that case.
So you can request your programmer for changing that client password and give a new password to the client.
Or if your program supports reset password functionality that client can reset the password(if the account is not deleted)
If the account is deleted you need to go your backups and restore that account data if you have backups for your DB.
Or if you have an admin account that has admin privileges you can view your client data there and if the client exists you need only reset account password.
So it depends on your program what functionalities you have in your program. In worse case ask your programmers, they can set a new password for the client(if the account is not deleted).