The can happen for various reasons when trying to upload documents:
- The file being saved incorrectly, e.g. the file type is wrong, the file size is 0.
- The file being open on your computer while you're trying to upload it.
- A document with the same name already existing for that student.
- Unexpected symbols in the file name.
- The file being corrupt.
Check that none of the above applies to the document(s). If you downloaded the file (rather than creating and saving it yourself), try downloading a new copy.