This is caused when the Primary site URL has a trailing space at the end.  It will appear as: / 

To correct this:
  1. Click on Administration 
  2. Click on Sites & settings
  3. Click on your site from the tree menu on the left
  4. Edit Primary site URL 
  5. Click Save

NetCommunity 6.64 has been updated to not allow a trailing space, but if it was there in older versions it will not have been updated automatically.