You setup a server to be a repo server. You've installed ftp and everything seems to be correct. You've also checked the permissions on all the files and they are correct at 755. Always test the ftp address in a browser. This error is deceving and it has nothing to do with repomd.xml. This has to do with the ftp address in your repo being wrong.
Using ftp the path should be:
ftp://x.x.x.x/pub/...../.../../... (anything after pub can be whatever you created)
The key here is don't forget to pub!
Additional info: Link