We have an intermittent error where a workflow attempts to edit an existing Excel workbook using excelservice.asmx OpenWorkbookForEditing. The error says "Someone else has the workbook checked out or opened in Excel; it can't be edited in the browser", but we have made absolutely sure that it is neither checked out nor open anywhere and still get this error. I don't think the Call Web Service action configuration is the problem since it does work for other items, but for what it's worth here it is:
The variable slt_Filename does not contain "?Web=1".
Not sure if you have resolved this or not. I had the same issue when trying to open and edit an existing Excel document. I found that you MUST close the document after opening in a workflow to ensure that it does not get locked. It is possible, that you have opened it in a previous workflow and never closed it - it remains locked. You must go in through designer or another administrative measure to unlock the file. Hope this helps.