Thank you for reporting this problem.
I have logged a bug report with the developers and hopefully it will be fixed in the next version.
Unfortunately, there's no workaround for this issue (the Email Button's 'body' content is not use-changeable and cannot be altered to something that Edge and IE11 will accept) and we might need to just wait until the bug is fixed.

Not yet.
You could override Juicebox's own click handler for the Email Button with your own (to use your own subject text) but I'm not sure exactly what Edge and Internet Explorer are choking on so you might encounter the same problem.
However, if you'd like to try it, you could use something like the following:

<script src="jbcore/juicebox.js"></script>
<script>
    var jb = new juicebox({
        containerId: "juicebox-container",
        showEmailButton: "TRUE"
    });
    jb.onInitComplete = function() {
        $('.jb-bb-btn-email').off('click');
        $('.jb-bb-btn-email').click(function() {
            window.location.href = 'mailto:email@address.com?subject=Gallery&body=Image No. ' + jb.getImageIndex();
        });
    };
</script>

This workaround uses the Juicbox-Pro API, specifically the onInitComplete event and the getImageIndex method (to fetch the current image number to be used in the email subject).

Thank you