How to replicate the issue:
Create a Page
Attach a spreadsheet in .xls '95 format made with either Excel or OpenOffice
Use the Excel macro to display the content
Click "Edit Spreadsheet" and see the sheet open in OpenOffice
Click Save

The issue:
Error saving the document TestSheet.xls:
Error transferring data to the internet.
Server error mesage: Internal Server Error

The only way to edit the file:
Mount confluence via WebDAV by:
Command-K
https://confluence.gsc.wustl.edu/plugins/servlet/confluence/default
change directories to the location of the test spreadsheet.
open TestSheet.xls
Save changes
The file may be read-only, or you may be tring to access a read-only location. Or, the server the document is stored on may not be responding.
Try Again
"TestSheet.xls could not be found"
Document saved successfully.
Cannot reopen the saved document due to low memory. Please close the document.

After that, the document no longer appears in the finder, but the changes do show up in Confluence.
You must re-mount with WebDAV in order to see the updated document.

So the big problem here is that the file cannot be edited directly on Confluence. The only way to edit the document is doing the procedures above.
It seems to happen just with old Excel Files.