Folders are case sensitive, so check that.
Pretty certain the leading \ in your sub directory will cause an issue
Confirm the username you are using is identical to how it appears in here within the brackets (Doesn't matter if it works on another machine "just like how you have it")
And look into your DNS settings, if the box for Append Device domain name is checked, un-check it and try again.
And don't work too hard on it if you are not on the latest firmware, you would hate to waste time on an un-fixable issue due to incompatible firmware
I am almost certain it cannot do SMB2 though, so if you patched Wannacry and then went and Disabled SMBv1 anyway, it may not work, in which case you would need to switch to FTP.