Hi,
Gave dropbox 4.0 a short check with Cubasis 1.9.6 on iOS 9.
File transfers in both directions work as expected.
Please double check if you have the latest dropbox update in use.
Could it be required in your case also to make dropbox aware of Cubasis, by transferring a file from Cubasis to the dropbox app first?
If the problem persists, please let us have a detailed step-by-step description plus device info.
Best,
Lars