concurrent upload in chunks

Can you clarify what you mean when you say "code is fine, and doesn't have any errors"? How have you verified that the code is uploading the correct data?

For instance, I don't see in this code where you're using "memStream", so it doesn't look like you're sending the actual file data.

Refer to the UploadSessionAppendV2Async documentation for information on the parameters it takes and how to use it.

Note that Dropbox will only save the exact bytes you send, so as verification I suggest comparing comparing the original file with the file uploaded on Dropbox to see if they differ. For example, you can take a hash of both files and see if the hashes match.