In my testing, smbclient (and cifs.ko as well) can not connect to the Apple SMB2/SMB3 file server in the most recent Mac update (OS X 10.10). Windows 8.1 and Mac clients can connect to their server. The file server advertises SMB2.02/SMB2.1/SMB3 in negotiate protocol (presumably no cifs anymore) and the failure cases are both NTLMSSP (in cifs.ko it is RawNTLMSSP and in smbclient it is NTLMSSP in SPNEGO). The failure is an NT Status logon failure on the 2nd phase of session setup (NTLMSSP_AUTH response). Windows and Mac succeed with NTLMSSP in SPNEGO with the same userid/password.
Although it used to work to Mac OS 10.9, now with connections to the popular update failing we will need to find a workaround if possible.
Scenario stopped failing after various reboots. Will reopen if recreatable.
For the archives:
Just ran into this the second time. Last time it went away like it did for you, but this time it wouldn't. After banging my head seriously against this and confirming that I couldn't connect to three different OS X installation (one 10.9, two 10.10), I somehow remembered that the host where I started smbclient was in a VM where the time often goes out of sync. After syncing time with NTP I could finally connect.