> "//server" is just the same as "/server" or "///server".

Repeated slashes aren't collapsed at the start of a Windows path. Here's what I get in Windows 7:

    >>> os.listdir('/server')   
    []
    >>> os.listdir('//server')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    FileNotFoundError: [WinError 53] The network path was not found: '//server'
    >>> os.listdir('///server')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    FileNotFoundError: [WinError 161] The specified path is invalid: '///server'