Do you have anything like a caching proxy in between the ePO server and the download site? It sounds like ePO is being given an older version of the file.
In the short term, you can manually download and check in the ePO package from http://www.mcafee.com/apps/downloads/security_updates/packages.asp - ignore the fact that it says it is only for ePO 3: it works fine with 4.x. That will at least allow you to update your machines while you're investigating the pull task failures.
No proxies. EPo->Switch->Firewall->ISP
One thing you can do to see if it's the ePO server causing the issue is to copy the contents of the HTTP or FTP site to a local FTP/HTTP/UNC location within your network, preferable close to the ePO server and specify that as a new source site and try to update from there, if the problem persists then it may be ePO.
Sounds like your license keys or Software Keys may be out of whack. Check your Software/Licensing in the ePO Server. You might also delete and recreate the repository.