TinyMCE Paste From Word Error


I am running WP Multisite.

On one of my client sites, when I click the TinyMCE Paste from Word button, I get the following error:

Unsafe JavaScript attempt to access frame with URL http://mappeddomain.com/wp-admin/post.php?post=3576&action=edit from frame with URL http://site1.maindomain.com/wp-includes/js/tinymce/plugins/paste/pasteword.htm. Domains, protocols and ports must match.

The issue is that the TinyMCE should be using the mapped domain (mappeddomain.com) instead of site1.maindomain.com. (I am logged into mappeddomain.com when this issue occurs.)
The correct URL should be:

I do not know where/how to change this so that TinyMCE uses the mapped domain? I am not using any plugins which modifies tinymce.

Thanks for you help!