Fatal Webdav Sabre\DAV\Exception\BadRequest: expected filesize X got Y

Of all the troubles I have sync’ing, I thought I’d attack each error message to figure out what they mean. I have these entries all over my log:

Fatal webdav Sabre\DAV\Exception\BadRequest: HTTP/1.1 400 expected filesize 10000000 got 2342912

What do they mean?