This has happened twice in our production system where the file created by our internal web serivce that wraps the Aspose functionality is returning an invalid document. These invalid documents cannot be opened in Excel and return the error "file.xls: file format is not valid.".
We are using Aspose.Cells version 126.96.36.199.
This morning after it created an invalid document, a few minutes later the same code was re-run and it created the valid document. So far it has only created invalid documents twice to our knowledge. If you still want to see the invalid document vs. the good document I can email them (attachment removed)
I did notice that if we try to open the file using the Aspose library that created it, it gives the following slightly more detailed error of:
Index and count must refer to a location within the buffer.
Parameter name: bytes