It appears that you may have tried to insert a new citation into an endnote field, rather than next to the existing endnote field. I would delete that citation group and reenter it, or try to unformat the citations (on a copy) and see if you can fix the embedded citation which might look like this
{author1, 2000 #RecNo; Author2, 2001 #RecNo; Author3, 2003 #RecNo {Blades, 1990 #25; Author4, 2004 #RecNo; (and so on with any remianing citations}
Change the curly bracket in front of Blades to a semicolon and reformat.
You may have to turn on CWYW from the third tab of the format bibliography window.
(p.s. You can always enter a new citation in front or behind the exisiting citation group, just make sure there is no space between that new citation and the existing field.)