The error comes from the server. Though it can be some sort of incompatibility. The server seems to be Microsoft IIS. Do you know what version? Also, did you try to upgrade WinSCP? You should in any case. WinSCP 5.5 is almost 8 years old!

Hi Martin,
During uploads I repeatedly get the error

Copying files to remote side failed - The specified data could not be decrypted

The data being transferred are just binary files and are not encrypted. The transfer appears to resume when I click the "Retry" button. I've tried enabling/disabling various options in Preferences without avail. Please see attached session log file. Appreciate your assistance!
Simon