When you use an FTP-over-Secure Sockets Layer (FTPS) client that implements Transport Layer Security (TLS) 1.2 to upload a file to a secure website, the file may not be uploaded as expected (fail to upload).
Additionally, you may receive the following error message:
550: The supplied message is incomplete. The signature was not verified.
This problem occurs because of a message-handling issue in TLS 1.2.
Some users of FileZilla say that they get a similar error to this, but switching to a different FTP client fix’s the problem. Some users suggest WinSCP works fine.
A supported hotfix is available from Microsoft Support. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing the problem described in this article. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.
To apply this hotfix, you must be running Windows 7 Service Pack 1 (SP1), Windows Server 2008 R2 SP1, Windows 8, Windows Server 2012, Windows 8.1, or Windows Server 2012 R2.
According to Microsoft the Hotfix is avaliable here ( https://support.microsoft.com/hotfix/kbhotfix.aspx?kbnum=2888853&kbln=en-US ) however when you check this link it says it is no longer avaliable, you may need to contact Microsoft directly to obtain this hotfix.