Marcal,
You are correct that the problem is in the server. The user is actually tracked in the SMP database and is stuck there. I don't know how to fix it, but I have heard others complain about this problem. People have hacked the derby database SMP uses to correct this as a temporary measure when it occurs. The next SMP patch should permanently fix this.
Maybe someone else here knows the steps to remove the user entry from the database?
Jason Latko - Senior Product Developer at SAP