It is in html and no warnings other than "input file not found"
Set $tempfiledir to $attachment_dir
$aspell_opts = "-a --lang=$lang --encoding=utf-8 -H"; // by FredCK
$tempfiledir = $attachment_dir;
It is possible that current directory is not writable and PHP tempnam function can't find writable directory.
If SquirrelMail configtest.php shows warnings (I am thinking about one specific warning), turn that setting off for SquirrelMail. spellchecker.php script has programming mistake which might create problems in some PHP configurations.
If you ask such questions on SquirrelMail mailing list, you would get same answers from same person and you won't anger hMailServer forum moderators.