Looks like one thing that results in this message is
https://github.com/winscp/winscp/blob/6.3.3/source/core/Terminal.cpp#L1554
https://github.com/winscp/winscp/blob/6.3.3/source/core/SecureShell.cpp#L1564
Based on that, it looks like its solved by connection or server side stuff.
I'd guess based on the short reply that it might have been a blip and solved without anything done by the user. Either that, or they're one of those that likes to leave it as a mystery :)
Tunnel errors that happens only once we connect to the local port (server-side forwarding problems). Contrary to local side problems handled already in OpenTunnel.
https://github.com/winscp/winscp/blob/6.3.3/source/core/Terminal.cpp#L1554
NET_TRANSL_CONN_ABORTED
is "Network error: Software caused connection abort"
https://github.com/winscp/winscp/blob/6.3.3/source/core/SecureShell.cpp#L1564
Based on that, it looks like its solved by connection or server side stuff.
I'd guess based on the short reply that it might have been a blip and solved without anything done by the user. Either that, or they're one of those that likes to leave it as a mystery :)