compatibility problem with "Send now" action

I have a personal plugin to manage metaboxes, which includes this particular jQuery line:

“$(‘.meta-box-sortables’).sortable({disabled: true});”

If I leave this instruction as is, a click on the wysija “send now” link on the admin newsletters list, will open the modal window as it should, but it freezes there: nothing is displayed and nothing is sent.
As soon as I delete this particular instruction from my plugin, everything is fine again.

What’s the connection between sortable metaboxes and this wysija action?

Thanks!

http://ww.wp.xz.cn/plugins/wysija-newsletters/