HTML code snippet causing editing to hang


Since adding a HTML snippet to my site, supplied from Mailchimp I cannot edit any block from the front end without the loader constantly spinning. I end up having to cancel out, refresh the page and then approve the changes.
Can anyone spot anything in the following that may not be playing nice with C5?

<!-- Begin MailChimp Signup Form -->
<div id="mc_embed_signup">
<form action="" method="post" id="mc-embedded-subscribe-form" name="mc-embedded-subscribe-form" class="validate" target="_blank" novalidate>
    <div class="mc-field-group">
    <label for="mce-EMAIL">Email Address  <span class="asterisk">*</span>
    <input type="email" value="" name="EMAIL" class="required email" id="mce-EMAIL" placeholder="Email (required)">
<div class="mc-field-group">
    <label for="mce-FNAME">First Name </label>
    <input type="text" value="" name="FNAME" class="" id="mce-FNAME" placeholder="First Name">
<div class="mc-field-group">
    <label for="mce-LNAME">Last Name </label>
    <input type="text" value="" name="LNAME" class="" id="mce-LNAME" placeholder="Last Name">

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JohntheFish replied on at Permalink Best Answer Reply

PS - to get back to a state where you can do this, you can revert the page version in the sitemap.
sarah3585 replied on at Permalink Reply
Excellent. Thank you!