Thank you for posting.
I don't have access right now to the versions you are using, but perhaps if we clarify the issue, we can narrow down the cause.
You've been able to do this before, so I trust that you are using the very same steps you were when you last exported successfully (checking the file type in the "save as" panel, for example).
One concern is the condition of your original data. If you have a backup copy of the file, you may want to try your export from that.
Create a new database file in FileMaker with a few test records, and create the Excel file from that. If the test file opens in Excel without an error, it's a good indication there is a problem with your original file.
You can copy the FileMaker Pro file and try opening it on another computer, but if you don't have access to another machine on which you can install the older version, that won't be an option. The conversion of the solution to a newer FileMaker Pro version is also something to consider. There are FileMaker Knowledge Base articles on file conversion to help, which you can find at these links:
With some additional details, we may be able to narrow down the cause of this issue.