So for some reason IE11 did not like this in my delete form:
<form action="scripts/sc-delete-user.php?&id=<?php echo $id; ?>" method="post" autocomplete="off">
I managed to workaround it by removing the form and using a button:
<input type='button' value='CONFIRM DELETE' onclick="location.href='scripts/sc-delete-user.php?&id=<?php echo $id; ?>';" class="button-link">
No idea why this became a problem after upgrading from IE10 to IE11.