This can occur when an FE user has a username that is greater than 20 characters in length. The Validation and Posting considers the Added by name should be less the or equal to 20 characters in length.
1. In General Ledger > Administration > Set up system security
2. Under users, locate the username experiencing the issue
3. Update the user name to 20 characters max and then save and close
4. The user must exist and sign out for the changes to take effect (and login with their updated username)