Any idea why Facebook is unresponsive when adding a page admin?

I want to admin a FB page someone else created. It was created without creating a profile first. So there’s a user/pass attached to the page and that’s it.

I logged into the account, opted to edit the page, and tried to add my own email address (linked to my FB account) as a page admin. I was prompted to type the page password, then click the save button, but then… nothing. The window went grey.

I’ve tried it a few times. Any ideas why I can’t add myself (or anyone else, for that matter) as a page admin if I am logged in as a page admin? Is it because the way the page was created?