Faced this issue while debugging another issue and was trying to login using SQLPlus. At that point of time, I had tried various steps to fix the original problem, and I was at a loss to figure out which step caused this.
From the error it was evident that the problem was due to some NLS issue and it is likely invalid environment.
I unset the NLS_LANG environment variable and then attempted to login. It worked perfectly.
PS: Do not forget to identify which script has this value incorrectly set and correct it.