Unexpected token ILLEGAL

I am unable to add any fields to new forms, or save forms. When I attempt to add a new field to the form, nothing happens.

Opening the Chrome inspector, I see on the console "Uncaught SyntaxError: Unexpected token ILLEGAL" in tw-sack.min.js

Can you shed some light on what the possible causes for this error are? I'm using the most recent version of Gravity Forms (1.6.11) on WordPress 3.5. I've disabled all other plugins as a troubleshooting attempt.

Thanks

Used Firebug with WordPress script debugging enabled so it wouldn't use the minified version.

Error happening here:

illegal character
this.xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
Line 43 of tw-sack

Unfortunately we have a similar issue. The only difference we are aware of between when it was working and now is that Wordpress was upgraded to version 3.5 using the auto update feature.

The issue (for us anyway) is that appears to only affect the form builder. If I click a button to add a field the following error message is shown in the Chrome developer tool console and nothing happens on the page:

Uncaught SyntaxError: Unexpected identifier (tw-sack.min.js:1)
runResponse (tw-sack.min.js:1)
xmlhttp.onreadystatechange (tw-sack.min.js:1)

The actual form on the web page works fine.
Let me know if you need more info?

We found out what was causing the issue.
We had the following function in a plugin file:

add_filter( 'gform_field_content', 'my_custom_field', 10, 5 );
function my_custom_field( $content, $field, $value, $lead_id, $form_id ) {
	$content .= '<div class="clear"></div>';
	return $content;
}

Commenting out this function fixed the issue. Not sure why but it works now. (Also, when we tested I'm guessing something was cached which is why we didn't notice a change which you've pointed out in your testing instructions).
Cheers.