The errors are caused by bug in FAR. When subdirectory size is calculated (F3), FAR asks plugin for contents of the subdirectory. As FAR plugin API is designed to hold contents of single directory only, this effectively causes WinSCP to release metadata of current directory. Once subdirectory size calculation is done, FAR eventually uses metadata of file in current directory again. But they are no longer valid, which may lead to various problem, ranging from incorrect behaviour of some functions to crash of FAR.
This was reported to FAR team years ago.
Workaround is to force reload (Ctrl+R) of current directory after calculation subdirectory size.