WinSCP Version 5.6 beta, Build 4526
Windows 7 x64 German MUI
OpenSSH_6.6p1-hpn14v4, OpenSSL 1.0.1g 7 Apr 2014
I got the shown error with the 5.6 beta when trying to drag&drop a file from a shown directory to a path in the tree on the left (I use explorer-mode).
This only happens when holding the Ctrl-modifier key to copy a file. After establishing a second connection the error occurs.