Javascript and Status=504 connection errors adding admins

Apologies if this is in the wrong space.

I'm the sole CUA for our SIs and I'm trying to add a few more administrators. In MOS I'm going to My Account > Manage Users, select a user, and "Grant Access..." in the lower pane. When the Grant Access window pops up I hit the radio button for Admin, then the double arrow to move all the SIs over to Selected Support Identifiers. Almost every time I receive and unresponsive script error or connection failure error and the browser crashes. The text of the errors and screens are below. The screens are in Firefox 41.0.2, IE 9.0.12, and Chrome