After the from Drupal 6.17 to 6.19, I was getting the following message on a multi-language installation:

warning: htmlspecialchars() expects parameter 1 to be string, array given in includes/ on line 857.

This is nearly the same problem as described in post.

Also this problem, was only when I was logged in or switched to the default language. The problem appears when not logged in and the language was set to the non-default language.

Fixing the problem

Open the file includes/ and search for the function check_plain.

One row of the function is changed. Here is the new function of check_plain. The bold parts are edited by me. So comment out the row with //Org, and add the row with //Fix

function check_plain($text) {
    static $php525;

    if (!isset($php525)) {
        $php525 = version_compare(PHP_VERSION, '5.2.5', '>=');
    // We duplicate the preg_match() to validate strings as UTF-8 from
    // drupal_validate_utf8() here. This avoids the overhead of an additional
    // function call, since check_plain() may be called hundreds of times during
    // a request. For PHP 5.2.5+, this check for valid UTF-8 should be handled
    // internally by PHP in htmlspecialchars().
    // @see
    // @todo remove this when support for either IE6 or PHP < 5.2.5 is dropped.

    if ($php525) {
    //Org: return htmlspecialchars($text, ENT_QUOTES, 'UTF-8');
    //Fix: return (preg_match('/^./us', (string) $text) == 1) ? htmlspecialchars((string) $text, ENT_QUOTES, 'UTF-8') : '';

    return (preg_match('/^./us', $text) == 1) ? htmlspecialchars($text, ENT_QUOTES, 'UTF-8') : '';

Remark: This is a temperary solution to keep your site running, it is not the definitive solution. The problem will be reported and than the quick fix has to be removed.