Can't send to remote site - 'Error transferring file [path inc filename]

I can't send any files to the remote site, though I can send files from the remote site to my computer. I get the following message:

Error transferring file [path and filename]
Copying files to the remote side failed.
Can't open that file: disk quota exceeded

This happens after I select files on my local drive and drag 'n drop them to the remote server. This is always how I've FTPd files using WinSCP.

I am using Windows 10
I am using the latest version of WinSCP, which I upgraded to after getting the identical error message using my previous version. I haven't used WinSCP for nearly a year and it worked fine then. I am using the same computer as then, and it has plenty of space on the hard drive.
I've never been given a disk quota that I might exceed.
I enabled session logging, but I can't see where any log file is stored, so I haven't sent it.

johnmacg wrote:

I've never been given a disk quota that I might exceed.

Well, obviously you have a quota.
Please contact your server administrator/provider. We cannot help you with server-side problems here.