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)