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.
First go to the Tag Groups settings menu and to Troubleshooting and System Information and check if the Ajax Test returns a green check mark. If not, you might find there more information where to search.
If that doesn’t produce any helpful error messages, 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.