Hi Knox777,
On doing a web search on "excel catastrophic failure error" it shows various links with various reasons like Excel version OR another instance of the file being opened as a background process OR some related to macro workbooks OR due to a control used on an Excel Userform, etc.
So I suggest to do the same and apply the changes/fix relevant to your exact situation.
You can post back what you come up with for anyone having the same issue.