Files with certain (long) file extension not transferred - Error 400

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.

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.