What is causing my sauerkraut to smell sweet?
In my experience a month is a bit long (although of course this depends on the size of the batch, etc) for active fermentation - however I don't think that's the problem.  In fact, I don't think there is a problem.  Since Sauerkraut is a 'wild' food, there are some variables that you can't (and shouldn't) control.  

The most likely (85% sure without knowing other factors) situation is that your kraut has picked up an uncommon variety of wild yeast and that yeast is doing its thing and putting-off this sugary/sweet smell as a byproduct of its processes.

Are you doing anything different with this batch that you've not done in the past?  Fermenting it in a different part of your house, maybe?  Is the temperature unseasonably warm/cold?  Regardless I think you'll be safe and come out with good kraut (which doesn't seem to be your concern) - but if you come out with REALLY good kraut, maybe you'll want to try to duplicate your experience :)