Is Multisite automatically creating a sub-directory?

I’ve had Multisite up and running quite happily for a few months.

In the past few weeks, however, I have had the following problem.

I have following sites set up as per example:

mysite.com
mysite.com/north
mysite.com/south
mysite.com/east

South and East work fine. However, every few days a ‘North’ sub-directory/folder is created in the root of my server.

So if someone accesses mysite.com/north they get a blank page simply saying Index of /north and I have to physically delete the newly-created North folder to get it to work properly again.

Then a few days later, ‘North’ will appear back on the server as an empty folder again.

Am I missing some obvious setup mistake here or a problem with my .htaccess file? I’m not particularly advanced with this setup but I have been able to learn enough to get it working – until now!