Unable to access samba share.

I'm setting up a ubuntu file server with samba shares and security = user.

These shares are able to be accessed and work correctly for both linux and windows clients. When I attempt to connect to the shares in OSX I get the usual username/password dialog box, however it doesn't accept the correct credentials.

I've googled for help for a few hours now, but cannot find anything that helps. I'm guessing its a security setting somewhere in OSX, but I'm not able to find out what.

Any ideas?