Unable to get a working directory for the file browser!

After updating a perfectly working system from 6.x-2.2 to 6.x-2.3 we get:

Unable to get a working directory for the file browser!

Module imce.page.inc is producing this erroneous error with:

function imce_check_directory($dirname, $imce = array()) {

  if (!imce_reg_dir($dirname)) {
    // HD: added this debug code:
    watchdog('imce', 'HD: Directory %directory considered inaccessible!', array('%directory' => $dirname), WATCHDOG_ERROR);
    // We get in here !? Why? This piece of code is new with version 2.3.
    // All our directories ARE accessible and writable and IMCE has been working just fine with version 2.2.
    // When I comment out the "return command" below, IMCE is working as before the 2.2 to 2.3 update.
    //return imce_inaccessible_directory($dirname, $imce);
  }
  .......

In the Drupal logs I now get for example: "HD: Directory 705 considered inaccessible!"
705 is a uid. All directories are named like that, there is nothing wrong with that.

What is function imce_reg_dir() doing as defined in imce.module? It is same as with version 2.2.

/**
 * Check if the directory name is regular.
 */
function imce_reg_dir($dirname) {
  return $dirname == '.' || (is_string($dirname) && $dirname != '' && !preg_match('@(^\s)|(^/)|(^\./)|(\s$)|(/$)|(/\.$)|(\.\.)|(//)|(\\\\)|(/\./)@', $dirname));
}

The problem is that imce_reg_dir() returns FALSE for a directory named eg 705 because of is_string($dirname). It returns TRUE for a directory named eg paul.

Well, our directory names are like 705, which are uids, and are working well with version 2.2