It's nothing directly related to Opus and it's not a 'normal' FTP error. For FTP, Opus simply reports any errors as from the remote server.
If I google this, I get 108,000 items on this 223 error. Something to do with Windows servers... I'd recommend you Google it to see what applies in your case.
It's not FTP - it's WebDav, and I'm using DOPUS to copy from a mapped network on my computer across to a folder on my computer. I get this error whether using DOPUS or Second Copy. It's clearly not an issue with DOPUS, but I'm not sure if it's something in Windows or on the server itself - Publications say it's nothing to do with them (of course).
Search for the error message. "the file size exceeds the limit allowed and cannot be saved"
The first two hits I get are Microsoft sites which look relevant. There is a registry setting in Windows that controls the maximum allowed size of transfers to/from WebDAV.