I'm using winbind with "winbind use default domain = yes". However, if the default domain is used, pam_mount does not have the information of which domain the user is from. I guess it gets the domain from the user name as there is no PAM_WINBIND_DOMAINNAME in pam data exports. In this case, %(DOMAIN_NAME) expands to a blank string.
Please, add the domain name to the export data in pam_winbind and make sure that it contains a domain name even in the case of default domain.
"add the domain name to the export data in pam_winbind" doesn't make sense to me. pam_mount looks at the user name and splits it up. Just don't use the evel "winbind use default domain" parameter.