krb5 error code 68

[Noolyg]

hi, 

I'm using MIT's kerberos and trying to check if a user is in the
kerberos DB.
The way i'm doing this is:
retval = krb5_get_init_creds_keytab(con, &creds, user_princ, 0, 0, 0,
0);
if(retval == KRB5KDC_ERR_C_PRINCIPAL_UNKNOWN) {
    // NOT_USER;
}
Is there a better way?

Sometimes i get the "KRB5 error code 68" for no known reason, and if i
try again after a while, i don't get it anymore.
Why does that happen, what does this error code mean?

Thanks a lot!