That sounds like the datalogger is bad.
While it's impossible to rule out totally, it's actually very rare for WL loggers to fail. What usually happens with download problems is that there's been some data corruption, which can happen either in the logger itself or, more commonly (and assuming that there's not a simple date/time synchronisation error), in one of the hard disk files that store the data on the PC (ie the wlk file or WL configuration files).
Most of the time, fixing the PC problem as suggested above will rectify the download (though I'd always recommend renaming the existing wlk file, eg to 2014-06.wlkx, rather than deleting it just so that you preserve whatever data you do have).
But if the various PC fixes don't work then the only ready conclusion is that the logger data is corrupted. For this to happen very occasionally isn't too surprising - it's no less vulnerable to corruption than any other type of computer/electronic memory and can potentially be hit by the usual culprits like mains spikes, interference from eg a mobile phone used very close by etc, even a stray cosmic ray, unlikely as it may seem.
The problem with logger corruption is that the only easy way to fix it is to clear the logger and that obviously results in losing any data that hasn't already been downloaded, which is why it's always worth trying the PC-end fixes first. But if nothing else is working then it's the only option. Simplest way is to change the archive interval in WL, accept the warning and then to change the logger interval back again to whatever is your preference.
