Indeed that error is mostly caused by firewalls:
https://winscp.net/eng/docs/message_transfer_channel_cant_be_opened

But if it started occurring after some successful transfers, it looks implausible. Unless someone changed some firewall settings meanwhile. Can anyone transfer files to/from that server from any other machine?

Seems this error below is fairly common and usually caused by firewall or NAT. I'm not experienced in resolving those issues and hope to get some pointers on what I should be looking for. I have attached log file with identifiable info removed, Thanks for any help.

It was working fine, as I had just pulled files down and it stopped working during transfer after many files had already moved. I have tried reconnecting many times and get the same error every time now.

Transfer channel can't be opened. Reason: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.
Could not retrieve directory listing
Data channel timed out.