Check that the chapter name that you are uploading the document to does not contain any of the following characters:
- Colons ":"
- Ampersands "&"
- Slashes "/" or "\"
These characters are used by the system and will potentially cause a conflict when trying to upload a new chapter. Simply rename the chapter and try the upload again to solve the issue.
Note that this issue can also be caused by large file size (larger than 50mb) and files which contain an extremely large number of text frames (>500).