It has nothing to do with "Sending special code: 12".
There's actually no indication what caused the error in the log. It can be a bug or something, but I cannot be sure as you are using an ancient version of WinSCP. Please upgrade to the latest version of WinSCP. If that does not help, please post a log file again, it will likely show more information than 4.3.3 log.
I have a WinSCP scriipt that runs weekly by a job scheduler. Approximately 4 weeks ago it began failing (non-zero return code to the submitting process) with the Special Code 12. I have not been able to identify any change to the WinSCP script or the WinSCP.ini file. Does the Special Code 12 in fact cause a non-zero return code from WinSCP? The explanation for 12 is Write protect The file is on read-only media, or the media is write protected. Is this referring to the permissions destination folder? Is there a switch or setting that will cause this to be ignored? I have attached the log. Any assistance would be greatly appreciated.