"550 Can't get filesize"

Smart FTP V 1.5, Windows XP, Norton Antivirus 2006, XP's firewall disabled

I'm having a problem when uploading files to my server, whenever I upload a file I get the error message that's named in the title (550 Can't get filesize). The file is uploaded anyway, but it seems as if the server can't recognize it, in this case it's the config file to WordPress which literally runs my whole site.

I've tried to CHMOD the files from 644 to 755 and 777, it didn't help.
And I have contacted my host about this, and the person I spoke to tried to access the FTP and upload files,
which worked like a charm, so it's something client based. But what?

Any ideas?

Hello ..

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