If you're getting an error referencing either of the above, we have a short term fix for you. It's caused because we reference a PHP extension that's included in all hosting environments.
You can contact your hosting environment and ask them to enable the mbstring php extension. Or you can roll back to 1.9.13 (see our rollback guide here: http://www.nextgen-gallery.com/how-to-rollback-a-version/.
But there's a relatively easy short-term fix for this if you are comfortable with FTP.
1) Unzip NextGEN 2.0 on your computer
2) Find this file:
3) Look for line 987, which looks like this:
$charset = mb_detect_encoding($this->root->plaintext . "ascii", $encoding_list = array( "UTF-8", "CP1252" ) );
4) And add a // to the beginning to comment it out, so it's like this:
//$charset = mb_detect_encoding($this->root->plaintext . "ascii", $encoding_list = array( "UTF-8", "CP1252" ) );
5) Save that file, connect to your site via FTP, navigate to the same folder in the nextgen-gallery plugin folder as in #2 above, and replace the copy of simple_html_dom.php with the version you just edited.
If you do this and it works, please confirm below. If you do this and have any other issues, please let us know that too.