This usually means that somewhere your blog outputs a warning or alert that interferes with the data transfer to your browser. In most cases it is caused by another plugin.
Try the following:
- Deactivate all other plugins and switch to a default theme. Then run the test again. If now all seems OK, re-activate them one by one (and keep running the test) to identify the one where it breaks.
- Add to your wp-config.php file the following lines:
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
Then run the test again and afterwards check the content of the debug.log file in the wp-content folder if there is any error message.