Can not upload file

Use of Sprint Nextel computing systems is restricted to **
** authorized use only. The use of any Sprint Nextel computing **
** system may be monitored and recorded by Sprint Nextel for **
** administrative and security reasons at any time. Your use **
** of these computing systems constitutes consent to this **
** monitoring. Sprint Nextel reserves the right to take **
** appropriate action against anyone who accesses or uses, or **
** attempts to access or use, Sprint Nextel computing systems **
** improperly or without the appropriate authorization. **
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**PLEASE NOTE**: Inbound files to this server MUST be placed in the /incoming directory

To me it looks like you cannot even login. Or do you mean that the authentication succeeds, but the connection is closed right after that?
Don't you have an access to the /incoming directory only? Did you try to specify that directory in session settings? https://winscp.net/eng/docs/ui_login_directories
If this does not help, please post a session log file.