Indeed, the file is excluded by the default file mask.
So I believe this problem should be resolved now:
Can't revert Synchonize UI include mask to default behavior

I created a session log where I created a local file, test_local.c, and tried to move it to the remote server with no luck. I then created a remote file, test_remote.c, which was successfully created on the remote server with some dummy content. Neither file could be dragged to the other server.

I did notice that my broken mask (the one that I can't delete) from my other thread appeared in the log; I have a feeling that solving that problem will solve this problem as well.

Please post session log file that shows how you cannot first upload a file to the server directly, yet immediately afterwards you can upload the same contents from the editor.

Anonymous wrote:

Creating a file which will be empty 0 bytes, and sending bytes are two different things. When something goes wrong, that can leave a 0 byte file around if no cleanup is done by the code.

I agree 100%. What I should have said is that I was able to create a file on the remote server by right-clicking on the remote panel and selecting "New", edit it with the built-in text editor and add content, and then save it. The new file, complete with all of my edits, was now sitting on the remote server. Yet I cannot drag and drop it back across to my local server. It acts just like it has completed successfully with no error popups, but 0 kB are transferred and no local file is present, not even an empty one.

The exact same thing happens if I create a file locally and try dragging and dropping it from the local server to the remote server.

Creating a file which will be empty 0 bytes, and sending bytes are two different things. When something goes wrong, that can leave a 0 byte file around if no cleanup is done by the code.

I'm just a guest. See what Martin says. It will most likely either be logs, or known issue.

Although I have been using WinSCP successfully with this particular site for several years, earlier today suddenly all files transfer "successfully" with 0 kB transferred. This is true both ways. However, I can create a file directly on the remote server by right-clicking in the remote window and selecting "New". I can edit files directly in the remote window and delete files in the remote window. I just can't drag files back and forth anymore, even after restarting the application and even restarting the machine.

I can also use a different local machine and connect to the same server with the same credentials using WinSCP and transfer files successfully via drag and drop, so I don't think it has anything to do with the remote server.

I don't see how to bring up a status window to see if there are any failure codes being issued by the server, or what the difference is between dragging and dropping a file vs. creating a new file on the remote server.