Failed upload bug?

When uploading numerous photographs by Drag and Drop, to a folder _Uploads on my server, After about 2 or 3 files have been successfully uploaded, the next finishes uploading, then prompts me for a password. After accepting the password, the file starts downloading again. after just a few bytes, another alert box appears saying "Invalid access to memory."

I am using Windows 7
WinSCP version 5.5.6 (build 4746)
Mode: Navigator
Using: FTP protocol
to: "/public_html/_Upload/"

Also, the internet connection I am using is very slow, measured in kb rather than Mb!

Thanks for your report.
I have sent you an email with a debug version of WinSCP to address you have used to register on this forum.