1) Are you allowing special characters in user names?
2) Did you just recently switch your database tables from ISO-8859-1 to UTF-8?

iconv has to do with multibyte characters. Does your new server have the iconv() library installed

Attached Files 20170116_11-35-17.PNG

Current developer of UBB.threads PHP Forum Software // 7.7.3 released!
isaac @ id242.com // my forum @ CelicaHobby.com