A backslash in linux is an escape character, so if you want to use that, you'll need two backslashes (one escaping the second). But I don't think that's the issue.
Just tried it here, from a VUDUO to a Windows 2012 server (which has the same IP and SMB stack as Windows 8), and it works without problems.
Since you're using OpenPLi 2.1, which has an older samba version then OpenPLi 3, all I can assume is that Windows 8 (by default) requires a higher protocol version then samba can support. You can try adding "port=445" to the "-o" part of the mount command, or else see if google knows how to enable smb over netbios on Windows 8...