user can not read/open e-mail

[erkac]

Hi,

one of my users has reported that he can not see some e-mails in e-mail folders and also he can not open some e-mails. The error message is:
Warning: mb_convert_encoding() [function.mb-convert-encoding]: Illegal character encoding specified in /home/services/webs/roundcubemail/program/include/main.inc on line 884

Thank you for any help.