If a document is stored in UR and editing externally, it will always make a copy of the file in the TEMP path and open that copy in the external application [1]. If a document is linked only, or if the linked URL of a stored document is edited externally, then it will open the document specified in the item's URL. In either case, UR itself will not keep the file open.
As kevina mentioned, if UR is also configured to show the document item internally (its extension is in Tools | Options | Browser | File extensions to display in internal browser view), then for a stored document, the copy in the TEMP path will be opened in the internal browser (and depending on how Windows is configured, IE may in turn host Excel to edit the document). So, it could be that an instance of Excel editing the document in UR's internal browser could be clashing with an external instance of Excel trying to open the same document.
[1]
http://www.kinook.com/UR/Manual/?edi...externally.htm