can't access samba share after upgrade to version 3.6.23-20.el6

Issue

After installing samba 3.6.23-20.el6 with yum update the users cannot access a share anymore. They get an error message "Group name not found". The share is defined by this block in /etc/samba/smb.conf:

[share]
        comment = Comment
        path = /share
        read only = No
        valid users = +DOM\Users +DOM\Admins
        force user = user1
        force group = user1

The user user1 and it's group user1 are local to the Linux system, ie. they are not AD accounts but defined in /etc/passwd and /etc/group.

When running smbd with log level 10 the log contains the following lines:

[2015/08/06 11:31:20.306688,  5] lib/username.c:171(Get_Pwnam_alloc)
  Finding user user1
[2015/08/06 11:31:20.306719,  5] lib/username.c:116(Get_Pwnam_internals)
  Trying _Get_Pwnam(), username as lowercase is user1
[2015/08/06 11:31:20.306745,  5] lib/username.c:149(Get_Pwnam_internals)
  Get_Pwnam_internals did find user [user1]!
[2015/08/06 11:31:20.306770, 10] smbd/service.c:548(find_forced_group)
  user1 is a User, not a group
[2015/08/06 11:31:20.306797,  3] smbd/error.c:81(error_packet_set)
  error packet at smbd/reply.c(803) cmd=117 (SMBtconX) NT_STATUS_NO_SUCH_GROUP
[2015/08/06 11:31:20.306832,  5] lib/util.c:332(show_msg)