Keep in mind that CIFS on AIX was never on the support matrix for 7mode. It worked, but was never qualified or tested. Support for it and Samba was always on a "best effort" basis. Only Mac and Windows CIFS were qualified.
http://support.netapp.com/matrix/

cDOT eliminated non-Unicode CIFS support for all SMB flavors. AIX would need to send CIFS requests in Unicode format to work. This is why Linux CIFS mounts work.
For example, in a packet trace of a Linux CIFS mount:
45 3.800625 10.228.225.140 10.63.3.68 SMB 148 Negotiate Protocol Request
1... .... .... .... = Unicode Strings: Strings are Unicode
46 3.801075 10.63.3.68 10.228.225.140 SMB 159 Negotiate Protocol Response
1... .... .... .... = Unicode Strings: Strings are Unicode
48 3.801209 10.228.225.140 10.63.3.68 SMB 324 Session Setup AndX Request, User: \root
1... .... .... .... = Unicode Strings: Strings are Unicode
49 3.814292 10.63.3.68 10.228.225.140 SMB 200 Session Setup AndX Response
1... .... .... .... = Unicode Strings: Strings are Unicode
AIX sends non-Unicode/ASCII and you'd see this:
333 2.402341 10.61.83.50 10.63.3.68 SMB 203 Session Setup AndX Request,
User: domain\administrator
- 0... .... .... .... = Unicode Strings: Strings are ASCII
On the cluster, in sktrace, you'd see this:
[kern_sktlogd:info:3069] 11.16.02.021841+0461 [1.0] CIFS_Dbg:nbb2e1c=CheckUnicode: Non-Unicode request not supported. cmd/sub:0x73/0x0<<<<<<
[kern_sktlogd:info:3069] 11.16.02.021847+0663 [1.0] CIFS_Err:nb8a3cb=ConvertInternalErrorToClientError: Client conn cid=55d4f69d at ip 10.61.83.50 getting default error from Res <-- the AIX client
The reason you see so much variance in SMB implementation is that there is no RFC standard for SMB, as Microsoft owns SMB and does not publish RFCs for it. So clients like Samba, AIX, etc get to implement SMB however they like. The fix for your AIX issue would be a patch for AIX that sends Unicode requests for CIFS over SMB1. This variance is the reason NetApp only supports Mac and Windows CIFS clients - it would be virtually impossible to support every CIFS client with all the variance out there.
NFS support spans so many clients because NFS follows RFC standards, so we can predict how the protocol will behave with accuracy.
Hope this helps.