Well I faced the same problem.
anyway it is not harmful since it is just warning
This happens when roundcube try to convert unsupported charset using
mbstring functions - for example windows-1256 charset-
For me I get rid of mbstring functions by commenting the lines
if ($MBSTRING)
{
$to = $to=="UTF-7" ? "UTF7-IMAP" : $to;
$from = $from=="UTF-7" ? "UTF7-IMAP": $from;
// return if convert succeeded
if (($out = mb_convert_encoding($str, $to, $from)) != '')
return $out;
}
in the file
/program/include/main.inc on line around 919
since most of my emails were encoding with windows-1256 charset
and depends completely on iconv converting function which will be the next in the code (make sure that your php is compiled with iconv support and the iconv module is installed ).
Maybe this is not the right way to solve the problem, but it works with me