Workbook Crashes On Open

Three times (by three different users) over the past two months, a workbook has gotten into what appears to be some sort of corrupted state. When you attempt to open the workbook you get the "report to Microsoft" screen which states "Microsoft Excel has encountered a problem and needs to close". Proceeding on this screen closes the entire instance of Excel, including the offending workbook and any others that are open. However, if I simply open and save the workbook in Excel safe mode, then it resolves the problem. But, again, it has happened three times so it keeps coming back.

A little more background: there is a custom add-in referenced which contains many custom functions. This add-in is also referenced by other workbooks, none of which have had this problem. The Workbook_Open event only does what I consider to be superficial processing; it hides some sheets, selects a sheet, defaults some shape colors, and defaults a checkbox to True. In other words, nothing too heavy going on at startup, in my opinion. I have tried several times to break into the code between the time the workbook begins to load and when the "report to Microsoft" screen is displayed, but this was unsuccessful.

I was hoping that the fact that simply saving it from safe mode resolves the problem might indicate the problem to someone on the forum. I have a copy of the file in its corrupted state but I have not attached it because it contains some business sensitive information.