Ehm – Actually I was using the AWS JS SDK, and it was a typo, i did not really intend to create /. But then I noticed the freeze ;-)

Maybe you can use a Lambda which writes on S3 to reproduce:
https://docs.aws.amazon.com/lambda/latest/dg/with-s3.html

How do you create such item? Amazon S3 console says

Folder names can't contain "/".

when I try to create a folder named /.

Since "/" is a valid name for a S3 Key , but it seems to crash WinSCP when having it , see screenshot.