I have 6 instances of WinSCP loaded, each one connected to a different session (two servers and three users on each server). Only one instance reported the error and unfortunately I didn't see which one. The problem seem to be related with a utility that resizes the windows of desktop applications, because when I executed the utility the error ocurred.
This utility is mine. If necessary I can find the API sequence used to resize the window.
Note: it always have worked well in previous versions of WinSCP and every other application I use in my development environment.