There are two differences: Your are uploading to a different folder. If you see the different behaviour even when uploading to the same folder, there's another difference: WinSCP uses strange Content-Type: application (Cyberduck uses application/octet-stream). WinSCP gets the content type from Windows API (which usually reads it from an extension record in Windows registry). Try fixing that or configure an override in Windows registry. See also:
Upload js files to S3 with wrong content-type
Let me know.

Sorry for the delay!
Yes, I just successfully transferred an .elfo file using Cyberduck to the same WebDAV server.
Logfile attached.

Thanks for your post.
Do you have any reason to believe with is WinSCP-side issue? It rather looks like the server is rejecting files with the .elfo extension. Or can you upload such file with any other WebDAV client? Please post its log file, if you can.

I am connected to a WebDAV server. In general, things are working fine.
Now, I am trying to transfer a file with the extension .elfo. The transfer aborts with the message "Error during transfer" (or similar, actual message is in German) and an error code 400.
Realizing that the file extension is longer than 3 characters, I changed the extension to .elf and the transfer works.
Then, remembering that there are regular file extensions longer than 3 characters, I changed the file extension to .xlsx and the transfer works!
Log file attached.