Hello :)
I've had the same problem with many other plugins. According to Gengo documentation that is because of incorrect plugin localization. And there's an easy fix. Here's the one for ShareThis:
1. Open up sharethis.php and locate this line
load_plugin_textdomain('sharethis');
2. Replace it with:
// a custom function for loading localization
function sharethis_load_textdomain() {
//check whether necessary core function exists
if ( function_exists('load_plugin_textdomain') ) {
//load the plugin textdomain
load_plugin_textdomain('sharethis');
}
}
// call the custom function on the init hook
add_action('init', 'sharethis_load_textdomain');
3. Save the file and try accessing your WP-installation again
4. You can also contact the plugin author to inform him of this incompatibility
I guess the problem is similar with MailPress, just look for the function load_plugin_textdomain
Good luck :)