Hi,
To enable logging of cifs "login" errors in the messages file you are first going to have to run 'options cifs.trace_login on'.
A little study of the file access documentation would probably be of use as well.
What sort of shares are you trying to access and how are you trying to access them? What does 'cifs shares' say about the shares that you are trying to access?
Does your filer have multiple hostnames or IP addresses? Multiple IP addresses in the same subnet?
There isn't a lot of information to go on here.