Unable to add covers to comics

In edit mode, I have the "Add a new cover" button below the primary cover. It lets me select or upload a cover, but when I click the green "Select Image(s)" button, the window doesn't go away and the page is never edited. All other editing capabilities are working as expected.

@Azuravian: I contacted staff about this last week, the bug has been changing slightly since so I believe they are working on it.