Thanks for your report.

Can you send me an email, so I can send you back a debug version of WinSCP to track the problem? Please include link back to this topic in your email. Also note in this topic that you have sent the email. Thanks.

I had two tabs open connected to the same host, uploading. This was the case all trough the night. In the night there is a regular short disconnect by the access provider to change IP address of my DSL brodband line. There was no problem with the tab that was active in the foreground. But when trying to switch to the background tab, that had finished it's uploading a while ago, the following error appeared:
"
WinSCP 5.5

Error message:
Invalid access to memory.
"
I guess, that the background tab had finished its upload before the DSL disconnect/IP change but had not refreshed the folder content display because it was in the background. When switching to it, it maybe tried to access the old folder content in the memory and mmaybe tried a refresh with the old connection before the nightly DSL connection change?
Anyway, I had to close WinSCP and start it again to continue uploading. Closing the tab alone didn't seem to get beyond the error. (latest Windows 8.1)