Description of problem:
realm join --client-software=winbind
creates and outdated Samba configuration. It adds the following parameters to the [global] section of the smb.conf file:
idmap backend = tdb
idmap gid = 10000-2000000
idmap uid = 10000-2000000
Those have been deprecated and replaced. Also it should use the rid backend for the domain it joined. The config should look like this:
idmap config * : range = 1000000-1999999
idmap config DOMAIN : backend = rid
idmap config DOMAIN : range = 100000000-199999999
where the DOMAIN is the domain name realm joined the machine too.
This needs to be fixed in RHEL7 too!